Hands close together or farther apart? Dominant hand underneath or on top?
This is just for an art exchange, but I’d like to fight ignorance too and try to look like I know what I’m doing. 
By the non-sharpened end :D.
Seriously, though, I don’t know broadsword, but I do know quarterstaff, and applying what I know from there, I would guess dominant hand on top (that is, closer to the blade), with the hands as far apart as the hilt allows.
What exactly are you doing with it? The grip will change a bit depending on the blow being struck, or the parry attempted, or whatever. Chronos was correct: dominant hand on top. Think baseball bat.
One guy is attacking in a standard diagonal swing, the other is parrying.
Huh, interesting. This is to maximize power, correct?
When I’ve fought with broadswords, I’ve had the most luck subduing them with a full nelson. Anything less, and the slippery bastards tend to escape.
Well the proper grip and stance would depend on the type of sword it is. “Broadsword” as a generic type is not a terribly useful descriptor in this case. But you might want to briefly peruse sites like this:
edited to add…
Try this one: A Brief Look at Stances & Guards of Medieval Longsword
- Tamerlane
Both, in genuine medieval combat you’d also grip the blade itself (you’d have gloves of course).
Find the nearest SCA group. Go watch a Fighter practice, ask them to show you.
Generally, a “broadsword” is a one handed wide bladed sword designed for slashing. The modern "claymore"with the basket hilt is a type of broadsword.
What are you imagining as a broadsword?
Sounds like a “two-handed” or “great” sword. In that case, your main hand is near the guard and your off hand is closer to the pommel.
In the huge zweihander swords, they even had a special area past the huge oversized guard that was covered in leather and had a mini-guard so that one could even sometimes grip it way up there.