Not *necessarily*, I'd say.
While, yes, the two countries have the same king, the opinion doesn't (just) represent the opinion of the king (as you don't play as a character in EU), but rather something like the opinion of the nation, or at least the governing elite (high nobles, members of parliament if a parliament of any kind exists, government officials, etc).
And the PU partner will have its own governing elite, who may have a pretty strong attachment to Rome - and be very, very opposed to the idea of the same reform being adopted in their country.