It would be super helpful if you could use one FX send to multiple FX racks. Good example would be, FX send 1 goes to rack 1 with a long delay and rack 2 with a short echo, both returning to separate FX returns.
I think the unique assignment is intended to improve clarity.
But for your specific requirement, however, you don’t necessarily need to use an FX send; you can also set up an Aux send for it.
The handling is exactly the same, but you can then assign multiple FX engines to this Aux.
As SQUser said, you can and should use Aux busses for this functionality.
In fact, I would say there is ZERO reason to setup and use FX busses - as in never do it. FX busses are simply a neutered buss, but still use up busses in the system. You can’t add any processing to FX busses like you can other busses, and as you found out their routing is also more limited. If you use regular aux busses instead of FX busses, you don’t have any of these artificial restrictions. In other words, there is nothing to loose by using auxes over FX sends, and a lot to gain by using auxes over FX sends/
Therefore I would suggest that in your buss setup, you choose “0” for mono/stereo FX busses and you instead use Mono Aux busses for this functionality as well.
PS- this brings up another point about the A&H consoles that is important to understand…. There is no need to use stereo sends/buses to your FX engines. Most FX run in mono, but even the “stereo” FX are simply dual mono. Therefore there is no need to “waste” precious buss capacity by using stereo busses to send audio to your FX engines. The FX engines have their own stereo returns which you should use, but they don’t use up any buss capacity. A&H doesn’t really advertise this fact, but it is likely why the “stock” settings have some mono FX busses setup, but no stereo FX busses set up.