You could run the mics into the Behringer but I'd reason to think the UA5's pres are much higher quality, no need to. The only reason I'd ever think of doing that is to roll off the bass from the stage rumble and bump the mids just a bit to bring some punch into the crowd mix.
What you COULD do -- and this is just a suggestion, but.. depending on the setup of the bands (and the willingness of the sound engineer doing the mix) take individual outs of channels that arent so hot in the mix (guitars usually, sometimes bass)... look for the insert channels on the board, take your regular TRS 1/4th cable, and insert it just until it clicks once, not twice. Then plug the other end in, and there you have a channel to add to the mix coming off the board to beef it up a bit in the behringer. You can also buy a more expensive insert splitter, they have them at Guitar Center for about $3 a piece.
Just remember, don't get fancy and pan shit around where it isn't in the original mix, or else you open up to phase issues, etc.
You can asjust the mix on the UA5 by the little level dial on the back end.
http://webpages.charter.net/jshambro/promo-t04.mp3 I was in complete control of the FOH mix so that explains why it works out on tape so well
Two mics on either side of the stage facing crowd, stage mix (Bass DI, acoustic guitar mic'ed with the new Taylor XPression System, vocals Neumann KMS105, crowd Oktava MC012) and then into a UA5 into a laptop, with a firewire drive attached.
Toured with that setup in one config or another for three months.
Made it easy to configure it for different rooms, etc... because I essentially had production manager duties on that tour, it made it easy to make a very HQ recording while doing FOH, monitors, sometimes lights, and supervising the local crew setting up for us and whatever headliner we were with, etc...