Hey - I like your thinking! That's fucking brilliant. Unfortunately the converter is only spec'd to 1 amp - probably not enough for both.
It generates a little heat just running the UA-5 with a 9.6v - not sure what it would do with more load. The JB3 pulls over 0.8 A when the hard drive writes and well over 1 A when it boots up. And any time it doesn't get adequate voltage/current, it just locks up.
Here's the
converter specs if you want to check it out