The ua-5 is digital OR analog record only, not both at the same time. There is a switch on the front that controls record mode.
You can, as Darrin posted, use the rca and xlr in's at the same time to do a matrix or 4 mic mix.
You would end up with 2 tracks with both signals, not 4 tracks though, so you have to monitor the mix on the fly.
It is not tough though, and there are threads here and on the Oade board describing the methods to mix with the ua-5.
JAson