Roger Cross (Curtis on 24) is on Continuum as Travis of Liber8 and Arrow as Detective Lucas of Starling PD. Supposedly shooting the pilot for a series based on the awful book The Strain.
From November of '07 to January of '08, Jessalyn Gilsig appeared in half-a-dozen episodes of Friday Night Lights as Shelley Hayes.
January of '08 also saw her appear as Gina Russo in episodes of Nip/Tuck, but never mind that now. What’s key is, she then spent April and May of '08 appearing in episode after episode after episode of CSI:NY as Jordan Gates.
I don’t think those are different seasons. If, however, they are, then she still qualifies, since she spent September-December of '08 reprising her recurring role from January-February of '07, as Meredith Gordon on Heroes. One way or another, that fits.
And before all of that, she was back to appearing in said Nip/Tuck role within a day of appearing as Lisa Rix on Prison Break from August to October of '05.
And before that, she spent April and May of '04 as Kelly Ronson on NYPD Blue, which you could argue isn’t the same season as her August-October '03 appearances on Nip/Tuck, in which case I’d point you to her June-July '04 ones.
Whoopi Goldberg appeared as Guinan on Star Trek: The Next Generation in February of '90; in March and April she (a) appeared as Brenda on Bagdad Cafe on CBS, and (b) of course kept appearing as Guinan on TNG.
Anyhow, she appeared as Brenda for one season finale in May before appearing as Guinan for the other season finale in June; in September, she was back for both season premieres, promptly followed by half-a-dozen more episodes of each.
Note that in July and August everybody was watching her earn an Oscar in Ghost.
Odette Annable appeared in just over half-a-dozen episodes of Breaking In during April and May of 2011 as safecracker Melanie Garcia; by October, she’d started appearing as Doctor Jessica Adams for the final season of House, but suddenly Breaking In was back on the air for its own final season.
So after four episodes of House in February of '12, she did three of Breaking In in March, and back to House in April and through to the finale in May, and back to Breaking In a week before its finale in August.
Fred Armisen was simultaneously on SNL and Portlandia, on different coasts no less! It probably helps that neither show was exactly a weekly.
Maggie Lawson’s on Psych and was in 11 episodes of Back in the Game, which was recently cancelled.
Obba Babatundé appeared in a couple of second-season Half & Half episodes, and then showed up for a couple of only-season Karen Cisco episodes, and then back to Half & Half, and then back to Karen Cisco, and then back for yet more Half & Half episodes before the season finale.
David Hasselhoff was on Baywatch and Baywatch Nights simultaneously. Other people (like Angie Harmon and Donna D’Errico) were also on both shows, but probably not the same seasons.
Danny Strong went from playing Jonathan Levinson on Buffy The Vampire Slayer to Marshall Gasner on Clueless and back and forth and back and forth and back and forth during season two of both shows.
He then came back for episode after episode after episode after episode during season three of each show, by which point a scheduling change meant he’d now appear on both on the same day.
Jason Alexander appeared on Seinfeld while voicing the lead character on Duckman.
I’m not sure Giancarlo Esposito counts. His part on Breaking Bad ended before Once Upon A Time started, and since Revolution has started he hasn’t really been on OUAT (he’s credited in two eps so far this year, but it’s like sub cameo level type stuff).
Hank Azaria and Yardly Smith were both simultaneously doing their voices on the Simpsons while being regular cast members of Herman’s Head.
Tahmoh Penikett was on both Dollhouse and Battlestar Galactica at the same time. Literally the same time. As in you could be watching Dollhouse as Fox was airing it (no DVR) and flip over to SciFi (or were they Syfy at that point, stupid name) and see BSG.
Kristin Kreuk was in every episode of Edgemont during its first season. She then continued appearing in the role of Laurel Yeung for the remaining four seasons of that show’s five-season run – while simultaneously appearing as Lana Lang in every episode of Smallville’s first four seasons.
(Okay, not “simultaneously” in the Penikett sense; just insofar as the episodes would air during the same months, often on back-to-back days. But that’s still pretty good.)
In January of '88, Lance LeGault appeared as Alamo Joe Rogan on Werewolf the same week he reprised his long-running role as Colonel Buck Greene on Magnum PI; he was then back as Rogan for more Werewolf episodes in February and March – and then back as Greene for more Magnum episodes, including the series finale, in May.
Also in May: his last appearance as Rogan, in the series finale of Werewolf.
Greg Grunberg played Sean Blumberg on FELICITY from '98 to '02. So in May of '01, when he was in a few episodes of NYPD BLUE as Joey Schulman, he was of course in four episodes of FELICITY – and when he was in a dozen first-season episodes of ALIAS as Eric Weiss from October of '01 to May of '02, he was of course interspersing those appearances with well over a dozen final-season episodes of FELICITY.
Christine Baranski - Diane Lockheart on The Good Wife and and Leonard Hofstader’s mother on "
the Big Bang Theory"
Nadine Velazquez is on The League and on Major Crimes. I think every show should have her on it.
Note that Sheppard has from October through December been back for episode after episode after episode as Curtis Hagen on White Collar and episode after episode after episode as the demon Crowley on Supernatural – after first appearing in both roles within a month of each other back in '09.
At that, Rob Lowe appeared as Eddie Nero on Californication in January and February and March of '11 – while making half-a-dozen Parks and Recreation appearances as Chris Traeger. (He was back in those roles on both shows in '12 and '13, as well.)
I did not yet notice a mention of Natalie Zea.
Fairly large-ish roles on:
“Justified”
“The Following”
and some work on “Californication”, and “Under the Dome”. I haven’t seen these last two, so I don’t know how much screen time she had.
It looks like many of the episodes aired during same or overlapping years.
I’m not sure but I think Jerry Stiller was on both Seinfeld and The King of Queens during the same season.
Continuing the short season cable drama/network show combo, Natalie Zea is a regular on both The Following (Fox) and Justified (FX).
Combining Saturday morning cartoons with broadcast, Jim Backus had a season or two of overlap doing the voice of Mr. Magoo and being Thurston Howell on Gilligan’s Island.