HF radios tuned to the same frequency
Hello, I do product line engineering support for airborne HF radios and the previous posters have it correct. It will depend on the exact equipment type and installation but the desired state is the have the "off-side" radio off the antenna when the "on-side" radio transmits. This would be advised even for dual systems with separate antennas unless the installation accounts for proper antenna isolation. Some equipment support simultaneous operations (SIMOPS), however, these tend to be military high power rigs.
I'll disagree politely on the "modern" as these radios are now software defined and use is much easier if Automatic Linking Establishment is used. It's a whole lot cheaper to use than that expensive satellite time! :>