You can sync two Things instances between two Macs easily by copying ~/Library/Application Support/Cultured Code/. But this works if only the donor instance has diverging changes. What if both instances have unique new items?
Bring up Things on Mac A, sync iPhone, shut down Things.
Bring up Things on Mac B, sync iPhone, shut down Things. Now iPhone has items from both Macs, but Mac B doesn't.
Pair and sync iPhone to Mac B again.