This is a tough one and it will be very difficult to solve without sitting in your environment and experimenting. Some initial thoughts include:
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Put a latch on your rule that processes the ON/OFF commands from the two Items configured so it ignores the command caused when you send the command to the other switch to update its light
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Use a timestamp or persistence to ignore any commands that occur too soon after one has been processed (like a latch but using a timestamp)
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Add logic to your rule to ignore any commands received that put the two switches into the same state. For example, in the default state both switches are in the same state. You touch one to turn on the light which triggers a rule. The rule now sees that the two switches are in different states so it sends the command to the OFF switch to turn ON. This second command triggers the rule again but now both switches are in the same state so ignore the command.
This last idea is probably the “correct” one.