Vertical or horizontal separation is needed for masts with multiple antennas. The required separation is dependent on what type of frequencies are being used and how the radios are programmed (Eg. unlicensed access point).
If both antennas on a mast are connected to remote radios, then a minimum of 2-3 feet of vertical separation is typically OK. If one antenna is connected to an access point radio, then it will be transmitting much more, and thus interfere constantly with the other. In such a case, a lot more separation would be needed.
Looking at the below graphic taken from the internet you can find the 850 MHz line and interpolate up a bit to get 915 MHz for Trio K and J series 900 MHz radios. When trying to avoid interference with access point radios a good rule of thumb is ~60 db of isolation, AND the two radios sitting on different channels. Nothing can be done with spread spectrum about having them on different channels. With 67 channels, odds are the radios will be on a different channel. That reduces the interference somewhat. But still on top of that you still want 60 dB more isolation. Follow the 60 dB line sideways until you reach the 915 MHz point. This will be ~75 feet.
The same basic rules apply for licensed 450 MHz radios. They have to be on different channels (preferably 5 MHz or more apart though) AND you need 60 dB isolation. Follow the 60 dB line across until you hit the 450 MHz line and you’ll see the necessary spacing is about 20 feet.
Consideration for the mast itself should also be reviewed. There should not be a lot of concerns with a YAGI antenna, unless it is being mounted next to a large tank type structure. 70cm - 1m or 2 -3 feet should suffice. An OMNI antenna however, must be mounted a few wavelenghts away from a mast. An example at 450MHz would be 1.5 meters (5 feet). At 900MHz it should be at least 70cm or 2.5 feet.
For vertical separation (with vertically polarized antennas) you’ll see the required separation is about 10 feet to get 60 dB isolation at 915 MHz. If you look at the horizontal separation table you’ll see that about 100 feet horizontal distance is required to do the same thing. Clearly, with vertically polarized antennas vertical separation is 10 x better.
If both radios are Trio masters you can configure them for Multi-Master Sync mode and they can either share one antenna (with a power splitter) or have two antennas at similar heights. Just keep one a bit above the other (eg 2-3 feet) to avoid heavy coupling from one antenna into the other.