My brother, who had a taste for rather stupid women at that point in his life, once convinced his date that he was both himself and his own twin brother.
He’s a strange man.
Though I still say you should honor your commitment to the first one you asked out, I am willing to concede and say don’t do both. Which ever one you choose, well, that’s your decision based on your level of horniness versus your desire to go after a possible long term relationship. That, I leave to you. Just one word of advice: which ever one you decide, don’t go poo flingin’. That would not be good in either case. 
If not, is your last name Brady?
I say don’t try for the double date. No matter which one you go out with first, you may find out that you are actually, you know, having a good time and don’t feel like leaving her in the early evening.
Me, I’d put the long time friend off until Saturday or something. You already pretty much know what she’s all about. The woman who’s been divorced for two years, you may find her to be wonderful. Or not, but under any circumstances, there will be too much stress about how to gracefully end the first date early enough to be on time for the second one.
In the long distant past, I did “double date” a few times. But this always involved the early date having an 11:00 curfew and the late date working a job from which she couldn’t leave until around midnight. That way times were set and definite, and there was some reason besides telling the early date “um, gotta’ go” too early to be reasonable or telling the late date, “I’ll pick you up at 11:30. Takes me a long time to get dressed”.
Hey! Check out the Google ad for this thread. “Love is Complicated”. Heh.
I’ve got “Live Psychic Readings.” 
OOH! Use love spells to get your boyfriend back! Better be watchin’ out there **MonkeyMule[//b].
OK guys, I know you were just dying to hear about my weekend. And since i have a sore back (UNRELATED) I’ll be brief.
I took the combined advice and did the two day thing. Girl number 2 (who was first) went with me to MoMA and then we went out for sushi. Good time, about what i expect.
Girl number one flaked claiming today that she had a bad haircut and didn’t go outside. Whatever. Anyway thanks for the advice and the next time the devil on my shoulder starts making sense I’ll ask the dopers to straighten me out