Jsn, you can do just fine with two good antennas with good quality feedline with as little as 10 feet of vertical separation at 5 mhz frequency spacing.
A quick look at the chart within the ARRL FM & Repeaters Handbook shows that you can achieve 75db of isolation with just 20 feet of vertical separation at 450mhz, more than enough for duplex operation. I personally have built UHF systems which use two antennas with close spacing and they all show no significant desense and work just fine. 20-30 watts is a good power level for this..... - Darrell/KA7BTV
