Any reason why FBM twin xl is 39" wide, i would like to split a king. but 39 x 2 = 78 not 76 like it should be. Does anyone know if I call they can resize it?

Try calling them.  Phone: (586) 627-3626

 

The worse that can happen is that you cut an inch off yourself. 

Keep in mind that latex can flex and stretch. 

Are you getting a cover from them too?  Ask them the dim's on that.  btw, my SleepEZ cover is 38" wide.  I think the foam can be 39" if layed out.  Sometimes you smush it a tiny bit to get it into the cover. 

Just make sure your vendor sells a good foam and cover combination that fit will together.