Duplex operation with multiple transmitters and/or multiple bands can work, but it leaves you open to intractible desense problems that can be impossible to cure. The issue is low level mix products that are normally undetectable can manifest themselves on receiver frequencies that can't be filtered out.
Run a set of intermod calculations with a single transmitter frequency. Then do it again with the second transmitter. Add other on-site transmitters, and run it out to 5th and 7th order mixes. At some point, you realize that everything mixes with everything else, and mix products mix with other mix products, and it becomes amazing that anything works at all.
You might get products at -95 dBm that are too weak to radiate, but since they "live" in the antenna and feedline used by the effected receiver, you can't filter them out, if they fall on the receiver frequency.
In my opinion, this is why some people insist it works fine, and others insist it doesn't. You pay your money, you take your chances. Try it. If it doesn't work, just run separate antennas. Don't waste too much time trying to make it work. Especially if you don't have a clear and precise understanding of exactly what the problem is.