no catch?
 

Receiver goes up for pass, catches the pass well in-bounds, but defender comes down with him, both have their hands on the ball, while going out of bounds.
Is this a catch or not?
Thanks in advance

Honestly need more information. Was anyone touching the ground in any way? Was this all in the air? And it could be a catch, but for whom would get it would depend on who did the things first or simultaneously. I would need more information to know when players had the ball and who hit the ground or how. You say well inbounds, but not sure that means a player was touching the ground or in the air?

Peace

Ok, I'll try.
Offensive player catches a high pass along the sideline (both feet inbounds). As he comes down with the ball, defensive player reaches in and has 2 hands on the ball along with the receiver. The two struggle for sole possession while going out of bounds.

Well it sounds like the catch is over. Just because they struggling out of bounds, the catch appears to be made already. Might have to see it, but not sure how a defender out of bounds changes the status of the catch. And simultaneous catches go to the offensive player or the receiving team.

Peace

The catch is made and the ball becomes dead when the player on defense touches out of bounds. The only way that player could secure possession for his team would be if he were the last one in sole possession before going out of bounds. Otherwise the last team on offense retains possession.

You don't need solo possession for a catch. The loose ball is not extended until the player touches out of bounds, just because possession is simultaneous. If they break the plane of a goal line with the ball together, it's a touchdown if it's the attacking team's goal line, not a touchback.