If the 18 and 27 hp engines have the same transmission
and props then the boat should go straight with the throttles set to the same RPM
on both. The 27 hp will loaf a bit and the 18 hp will work harder. Nothing wrong with that.
So center the helm
and rev up both engines to the same rpm
and see what happens.
If the 27 hp engine has a lower gear
ratio or a prop with more pitch
then it will be working about as hard as the 18 hp engine and the boat will need some offset helm to go straight. Not a big deal either.
FWIW, my Island Packet
Cat 35 had two 27 hp Yanmars and was not overpowered. That may be why the PO decided to upgrade one and presumably the second later.
Can you ask him?