I have a very similar setup here, vhf/uhf omni for general reception and an 800 mhz omni for trunked P25 stuff. I tried searching for a duplexer to combine the specific frequencies I wanted to receive and no luck other then overpriced stuff. I ended up getting a combiner to do the thing I needed. BUT....what a lot of people dont know or refuse to acknowledge is that a good quality 2 way splitter will combine 2 different frequencies. Good quality is not radio shack or walmart/dollar store junk. Good ones are online and have a spec sheet available to view to see insertion loss, attenuation, etc
I have an RSP-1A that I used to check incoming signal levels on the vhf/uhf omni, and then the 800 mhz omni. Then combined the two antennas using a backwards 2 way splitter and then checked again. Only difference is the 3.5db in rf signal loss that you would get from the splitter, but by going by the received signal graphs on sdruno and audio on both analog and digital decoding, there is negligible difference between standalone and combined signals. And the system is a combination of 50 and 75 ohm coax and components, 0 reception problems mixing the two.
With the distribution setup I have here for the multi scanner system, I can receive 50+ miles out, more depending on atmospheric conditions. I use the same type of setup but different antennas for my ATCS hosting site using a backwards 2 way to combine two 900 mhz yagis pointing north and south, and it works perfectly for both directions.
So I have no reason to look for a $100+ duplexer to combine signals when a $7 antronix 1 ghz rated 2 way does the job perfectly well
