WRYV757

If you use the programing software you can add channels with frequencies you want and put "ADD" or "Delete" in the "Scan Add" column and those channels that have "Add" will be in the scan list during scan mode.

My opinion is go with the duplexer, its simpler solution. yes there will be loss, a well tuned Q circuit 6 cavity duplexer will be about 2.2 db ish loss at 450-470 mhz. Lets say a 7.4 db gain antenna - 2.2 db = 5.2 so still considerable antenna gain and higher the antenna the better. Its just UHF ....loss is inevitable..you just minimize it as much as reasonably possible.

You could try co phasing antennas vertically, one at the the height you know works for the one repeater and the the other at the other available height. The antennas would need to be the same and the harness feeding the antennas would need to be 75ohm. The distance of each feeder cable would need to be equal and in odd 1/4 wave lengths. just a thought and it would give added gain vs the single antenna if done correctly

You could do a stealth antenna placed on your roof vent of your home. They make one that does not require a ground and that would be a simple option and have some elevation. https://www.ruggedradios.com/products/uhf-stealth-antenna?variant=39745864433760