No problem having more than one USB mouse connected but you could have conflicts between the drivers or the control panel software. That happened to me when I tried to install both Microsoft Intellimouse software and Kensington Mouseworks software. I had better luck with Logitech Mouseman + Microsoft, but basically it seems that only one control panel package can be operative at a time. You can, however, install the drivers only or even just use the Microsoft drivers. See my post in this forum:
That pesky Logitech Mouse button #4
I currently have three different brands of mouse installed on a Win98se system:
Microsoft Optical - USB
Logitech Dual Optical - USB
Kensington Trackball - PS/2
The full Microsoft drivers and software package were installed. The Logitech mouse was installed with just the Logitech drivers. The Kensington trackball was just plugged into the PS/2 port and recognized as a "Microsoft PS/2 port mouse". It works just fine with the Microsoft drivers.
I recommend you have a full system backup or at least some kind of rollback software running before you start experimenting with these multiple device configurations.