If you use 2 hand-crossbows/pistols, can with a single prepare maneuver recharge both weapons?

I'd say no, but you can allways take a fatuigue to make an extra manouver. Or I believe that you can take one extra challenge die on the shot if you do not take the time to spend a manouver to reload. Dual wielding (and dual reloading) should not be too easy in my oppinion.

You can't. It takes one manoeuvre to reload each firearm; taking more weapons will allow you to take more shots before reloading, but if you want to then reload all your weapons, it's going to take a while.