Was the High Adventure Cliffhangers: The Buck Rogers Adventure Game compatible with Buck Rogers XXVc game? From what I understand XXVc was a simplified AD&D 2e system.
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Looking around a little bit, it seems you're right- Buck Rogers XXVc appears to use a modified AD&D system (source).
However, High Adventure Cliffhangers appears to use an exploding d6 system (source - scroll down to the entry on High Adventure Cliffhangers). You can actually see a note to this effect in the RPGGeek entry for the game:
Those links actually imply the answer: RPGGeek doesn't group them in the same system family so, unless there's an error in the entries, they are very probably unrelated.