Yep, the lower one in screw direction and the upper one in un-screw direction with light force till they virtually form a unit.
WD40 is not the best solution for severly stuck or rusted connections.
It does serve a variety of puposes 'so-so' but isn't especially good in any apart from water displacement perhaps.
Liqui Moly MOS2 is by far better to get severely rusted or stuck screws loose.
Should you not get the bolt out with a force sensible enough to not risk to break the bolt,
you can give a light , exactly vertical hit to the top of the bolt to crack up the tense joint in the base of the thread a bit..after that ideally apply Liqui Moly MOS2 and let it sink into the microscopic cracks over night.
Just gentle knocks is enough to vibrate the base lightly to let some solvent move in.
Ofc you need to place the deck on a cushion before you give the gentle hit to the bolt, so no surface gets damaged.
The second possiblity...especially if someone really glued the thing in with bomb-proof screw-lock,
is heat.
So the last means and the best working with superglue or screw-lock ultra would be a blowtorch.
Direct only at the end of the bolt, the heat will travel up to the problem area by itself, no need to point towards the deck in any way.
Good luck..