The answer to the question "Would this fail" is always YES - no matter what the question is about. The real question is whether something would fail so often as to be useless.
Logical lesson finished 
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If any pair of transmitters on the exact same frequency (both within range of the receiver) transmit at the same time the receiver will get garbage. I am not familiar with the HC12 but I know the nRF24L01+ transceivers have an auto-retry capability. That is likely to make things worse, not better, because they will both be retrying.
If you have two transmitters on different frequencies (channels in nRF24 parlance) they may still interfere with each other if the devices are very close together. I have not tried but they might work if they use widely separated channels.
I don't know if the HC12 has an auto-acknowledgement feature like the nRF24 but, if it has, you will almost certainly need to turn it off with multiple devices on the same channel thus losing one existing layer of protection.
I can't help thinking that your idea introduces so many additional complexities that it is likely to reduce overall reliability. One of the problems you will face is finding a set of test situations (i.e. drone in one place and controller in another) that will properly exercise your system to the point where you can have confidence in it.
...R
PS. I was originally assuming that you are thinking of having 2 HC12s on your drone but on re-reading your Post I wonder if you are thinking of having an extra ground-based relay station so that either the relay station or your controller would be in contact with the drone.
If that is what you have in mind I think most of what I already said remains relevant. And the relay station would introduce the extra complexity of a slight delay in the propagation of the signal. That delay may not matter for control purposes but it could cause confusion when the controller (or the drone) is getting the same message twice at slightly different intervals.
It might be made to work if there was some means to switch between the two message routes so that they cannot interfere with each other. But I suspect that would be complex. Maybe the relay station could operate on a different channel and if the drone failed to get regular messages on channel A (from the controller) it would switch to listening on channel B for the relay station and vice versa.
...R