Countertop question

Just alter the miter angle as needed. More off the front gives a bigger than

90 angle. As long as you do the same amount of alteration to both sides of the miter they will line up fine. For large adjustments use a power plane. Always plane so the laminate is cut before the particle board. Or it will chip. For smaller adjustments use a belt sander. Again make sure the tools rotation contacts the laminate first than the particle board. A very slight back cut will give a nice sharp edge to joint. Don't overdue this back cut or sealing may become an issue. For extreme alterations you may also need to reroute the clamping pockets.

If I understand you correctly.

the slabs are say 24" wide. When you cut them at more than 90 then they pull away from the wall. So unless you cutting the slab to fit depth wise, I think your in trouble.

Dry fitting one to the other is the only way I know. Start with one see how it lays out. Then try the other as the first piece sometimes it makes a difference. Starting with the smaller of the two pieces goes better for me some of the time.