Hmm, if you were using xmame.x11 in the past, and had it working full-screen, I presume that you have modelines in your X configuration file? If so, have you checked the X logfile after your TwinView change to see if those modelines will work with your updated configuration?
If you want, just email me at
phulshof@xs4all.nl. That way if you need to send me some configuration/logfiles to have a look at you can.