I used to have a TSR-8. It's a great machine. From what you've described, you don't have a free track to "bounce" to since you've recorded on all 8 already. If you had a free track, you could take the two bass tracks into your mixer, adjust their levels, then output that submix to the available track, freeing up the two bass tracks to be recorded over with whatever you wish.
Of course you can't do this now because you don't have a free track on the TSR-8. I'm not as savvy on taking things into the computer (yet) and shooting them back out, but certainly if you have 8 available inputs on your PC or MAC, then you could output all 8 simultainously and record them into a audio sequencing program. Like I said, I'm not proficient in this method yet, but I believe you can even adjust the position of track forward and back in an audio sequencer, so even if you only had two inputs on your computer, you could still send tracks out two at a time then adjust their position to be in sync.
Of course, if you just have the standard PC audio card, the A/D converters won't be too hot (or so I've read, over and over) so you might be shooting yourself in the foot to start with.
My question would be....can you do without one of the bass tracks, or is it just essential to have stereo bass. What was different about the two tracks? Was one a DI and one a mic? Two mics in different spots?
I'm just getting back in to recording after about a 12 year absence, so I'm not really hot on todays methods and styles, but I rarely used more than one track for a bass. Of course I've mixed more than one signal for bass, but I've always put them together on a track. Certainly less flexible, but it help to get the sound you may be looking for.
I hope I've typed something that is useful to you...