You really need to think carefully about this setup.
Having two radios with minimal separation, on the same band, using separate antennas instead of a proper duplexer is going to cause a lot of headaches. Even with 4 or 5 watts, you will need a substantial amount of separation between the two radios to prevent the transmitting radio from causing desense in the receiving radio.
Single antennas and a tuned mobile duplexer are really the way to go.
My concern with the V71 would be as the receiving radio, it's probably not going to have the separation and the filtering to deal with a very close 4 - 5 watt portable transmitting. It might work better with the V71 as the transmitter.
As for the controller, you need to set this up to have the receiver audio trigger the other PTT. Since neither of these radios have a readily accessible COR. The 860 to 860 cables probably had a VOX circuit in them, but I'm not 100% sure. Either way, comparing amateur grade gear to commercial gear and trying to make it all work together is going to take some work. Ideally you might do better with a commercial mobile rather than the V71. Keep the dual band for QSO's.
I understand being limited by budget and the like. I'm sure this can be done with what you have, but really, consider a mobile duplexer and a single antenna. It'll save some headaches.