Later versions also carried 6 .50cal in the wings...
so in theory...unless the wing packs couldn't be carried on the versions with the wing guns...
You could have an 8 gun nose, 6 fifties in the wings, and an additional 8 fifties in two twin packs under each wing, for a total of 22 .50cal machine guns.
Now - it more than likely never happened, why would anyone want the added drag and weight of 8 more .50s when you've got 14 already...but...unless someone has a pilot's manual for an A-26, it could,
in theory, be done.
According to Francis Gallemi's Warbird Profile A-26B/C (Aries Publications, 1994), two twin 'tubs' could be carried under each wing. The author mentions that used in conjunction with the wing-mounted .50s, it dramatically increased the Invader's firepower...
Attached is a pic...