In the PSBT you need the shared_secret_tweak and also a way to specify which spend private key to tweak. For example, I am spending outputs sent to two entirely separate silent payment addresses in the same PSBT: I need a way to say “the shared_secret_tweak for this input is used to tweak spend key A and the shared_secret_tweak for that input is used to tweak spend key B.”
This is one I need to double-check: does the PSBT provide enough information about the prevout for us to ensure the output being spent is not a segwit witness version > 1, for example?