Getting dual screen games to work, under my technique, requires the most awful hack of the mame source code you've ever seen. The solution for now is to wait as Aaron has finally upgraded mame's video core and one of his priorities is multi-monitor managment.
My experiments with dual monitors came to a halt because I want to use psone screens and I couldn't get them to diplay properly. As of this morning, however, my matrox mystique vid card came in and my psone displays are finally working! Dualie (my multi-monitor-managment software) is back in business and I will be working on it actively again.
As for j5, just use the -position tag as mentioned in the readme.txt Setting a negative y value makes j5 move above yoru primary monitor, a y value larger than the resolution makes it move below the primary, ect....
You can have J5 constantly refresh the game by using the -listen tag. J5 "listens" for a text file named clo.txt in it's root directory. All you need is a fe (or a batch file that can be launched from a fe) that rewrites the command line you would typically send to j5 to clo.txt prior to a game launch.
As I said, my secondary monitors weren't really working before, so this mode hasn't had a lot of testing, but it should work just fine.