For an Airbus (any) with a dual installation, then it's fairly easy.
Airbus use a common antenna, the fin leading edge, certainly for 300 to 340. Where a dual installation is fitted then interlock is enabled, i.e. transmission is only allowed on one HF at once, the other being inhibited on a first come first served basis. An additional feature of HF interlock is that the non paired receiver is also inhibited. so both on the same frequency should not be an issue. This is normal HF practice.
For Boeings it can get more complex with differing installations. 70, 72, and 73 all used common antennas, the "HF probes" on the 70 & 72 and back to the Fin leading edge on the 73. In a dual installation all would have interlock. This is valid for the 5,6, & 7 also which use the fin (note sure about the 77 but visually seems a good guess)
The 747 classics (2/3) in most I have seen had a dual probe antenna installation from each wingtip trailing edge. I have seen these both with and without interlock, those without seeming to manage quite happily with the separation distance between the wingtips.
I have no knowledge of the 400 but expect that its back to the fin.
These things vary much from installation to installation so I have tried to reply in general terms but expect the howitzer.
