I think the clock issue is the least of your problem here but I'll explain it.
When you record something with an A/D converter there is a clock at (for instance) 44,100 beats per minute. Every clock beat you write 16 bits of information to the hard drive. The problem is that no two clocks are exactly the same (one clock runs faster than the other) and you have clock drift. I used to record to an H120 and an R09, pull up the 2 files on the the computer, and one is a tiny bit slower than the other and you have to stretch one or the other, which is a pain in the butt. Seth's point was that if you recorded with 2 applications (Audacity to record the V3 input, and Soundforge to record the UA-5 input) then you could theoretically end up with the same problem as 2 recorders. If they both standardized on the computers clock it wouldn't be a problem. But who wants to run 2 applications? What you want is one program that will look at 4 channels on the screen.
Most standard soundcards have 2 inputs. Others have 4 or 8 or 16 channels... all in one hardware box which connects to your computer via USB or Firewire. Most software can handle that just fine because it's talking to one hardware device. You tell the software which input tracks to record, and it works.
But that's not what you have. What you have is "a bunch of little soundcards instead of one big". It's like stuffing different soundblaster cards into the back of a PC's PCI slots, and trying to tell it which one to use. To software that is a HUGE difference. You have to find software that is happy with multiple soundcards. Such software exists but I'm not sure which ones.
One option may be to buy a device... I can't remember that name... but there were some devices which you plug into a computer's PCMCIA card and there was this octopus of cables for which you could plug in. Then you run V3 digi > 1 leg of octopus, and UA-5 digi > another leg of octopus and to your computer it looked like "one big soundcard", and then you can use Audacity. I haven't seen of of these things in a while. They were very expensive new, but they show up cheap in the yardsale on occasion.
You still don't have an answer, but maybe you understand the challenges and what to look for.