I've been using two Promise Ultra100 TX2 controllers together in my rig for about 2 months now without any problems whatsoever. I have 2 large HDs connected to one, each on a channel by itself, and a CD-ROM and a ZIP drive on the other, each on a channel by itself. On the motherboard I have a DVD-ROM and a CD-RW, each on a channel by itself. If you have ever used a Promise controller you are familiar with what it looks like during bootup, instead of showing four (D0-D3) channels it looks like one big controller with eight channels (D0-D7). I don't RAID (cause you can't with these controllers) and I did this setup not out of necessity but because I had an extra controller and was curious. I can say there are no conflicts and copying files from drive to drive is faster, most noticebly when I backup one HD to the other (Norton Ghost). The only thin to keep in mind is if you are going to hook up your HDs to a Promise make sure you set "boot from SCSI" in your BIOS.
Hope this helps.
Good luck to ya...