Mamushka, with the optipac in p1/p2 mode, and -multimouse enabled (set to 1), and -verbose enabled, does mouse show adding both mice? (The lines will start: "Input: Adding Mouse #x...")
OK you officially lost me
What is "-verbose enabled"? Please note, I am a Mechanical Engineer so I don't understand much beyond "righty tighty, loosey lefty" 
Ah, forgot you're using mameUI which doesn't have that option in the UI. The -verbose option is a common option for commandline apps to "print out lots of info to the commandline,", but mameUI doesn't output to commandline, either. Makes it a little harder to see what mame/mameUI is seeing.

You could either download the commandline mame, or run mameUI from the commandline.
To run mameUI from commandline (note I don't have Vista):
-Open the mameUI folder in windows explorer.
-Highlight what in the address box, and copy (ctrl-C) it.
-Open a command prompt (I think start -> all programs -> accessories -> command prompt, but I don't have Vista yet, or
read this maybe if you can't get to the command prompt)
-Type "cd " (including the space), right click the mouse in the command prompt windows, click paste. The address you copied earlier should appear.
-Press enter. You should be in the mameUI folder.
-Type "mameui32 -verbose
marble > verbose.txt" (
marble can be any TB game you have.)
-Start game, and exit (esc key).
-Open the resulting file verbose.txt (IOW double click the file that just appeared in the windows explorer.)
-Look for the lines I mentioned in earlier post.
And since downloading normal mame will probably take about the same steps + the setup, I suggest trying the above first. I know the above works for