The only thing I would add to Hans Meiser's answer is the possible use of an expander/gate.
If the target conversation is the focal point, then it should be slightly louder than the rest(at certain frequencies), allowing you to use an expander or gate to emphasize this conversation. You may find it tricky though, If you aren't an audio professional.
You would need an expander or a gate with internal(or external) sidechain capabilities (Example).
You could then find the frequency(or frequencies) in which the target conversation is dominant, and feed that refined signal to the sidechain.
Then, with some adjustments to the expander/gate's parameters, the result should be a signal that gets louder and quieter with the target conversation's voices.
In practical application, this could be tricky to maintain. It could take a bit of work.
What is an Expander/Gate?
Apple explanation of Expanders/Gates
Pro Tools Tutorial: Sidechain Techniques
Side-chaining in Cubase