As far I know, in the time of the MIR + Space Shuttle dockings it was a problem to solve.
Are their docking systems compatible?
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One of the docking systems in use on the ISS is a passive APAS-95 system. Tiangong-2 is believed to also have a passive APAS-95 system or something mechanically compatible. In order for two APAS-95 adapters to connect, at least one of them needs to be an active adapter, so Tiangong-2 can't dock with the ISS, but a Shenzhou spacecraft could probably dock with either.