There should be weatherstripping around the door perimeter and where the two doors meet. If yours is missing, replace it.
If you know the door manufacturer it will be easier. Otherwise you might need to find something close to the original.
Just before the door closes, look down the door edge and jamb, that should be equal distance. If it is not, the bottom rollers are adjustable to make things level/plumb. On some doors the adjustment is on the side behind small plastic caps. On others you have to adjust from the end of the door. That should solve your problem.
And as suggested above, check that the weatherstripping is OK, not torn or missing.