I don't think it would be too bad, unless you are planning to leave it for your wife/kids/other uniterested person to use while you are gone. When I watch my Uniden, I always have the Mercury II turned on anyway. I move the dish, and set the polarity with the Mercury II, and then watch the Uniden. I assume you are planning to use the Uniden for sports, and finding things will take a little bit of experimentation anyway. Once you get used to it it won't be too bad.
I don't know about connecting the wires from a Vbox and the Uniden together. You would be connecting two power supplies together, and would probably burn up at least one of them. If you don't want to use the Uniden to move the dish while watching the Mercury, it would be best to move the wires from the motor back and forth between the Vbox and the Uniden. That would probably be more of a pain than turning on the Uniden, moving the dish, and turning it off.
Another thing, if you slave the Uniden to the Mercury, install a DC block between the Uniden and the Mercury. I haven't had the problem, but I have read that some people have burned the IF output from their master receiver when they plug in the slave receiver without the DC block.