I'm replacing my Cal-King mattress.  I was wondering if it would be a better idea to stick two extra-long twins together and put a cal-king size topper on it so I wouldn't feel the crease in the middle.  The reason why I think this might be good because maybe the size of the king mattress causes there to be more sag in the middle as time goes by.  Maybe the narrower twins won't sag as much because there would be less width for sagging to occur?  Can anyone help with this decision?

You will still FEEL the gap in the middle. As long as you don't plan on laying near the middle, it will be fine.
My wife and I sleep on 2 twins because she's got restless legs and I'm a light sleeper. We just  "visit" each other at will, and it works out better that way for us. (Otherwise I'd get no sleep!)

I see.  Well, I'll probably stick with the Cal-King then.  If I could find an 84" long twin, I might consider it again.  I'm tall so the extra 4 inches are helpful.

I believe x-tra long twins are 80 inches long--hence their ability to form a king bed (using two xtra long twins). And you can use something that 'bridges' the gap between the two...I've seen it at Bed Bath and Beyond, and also FBM has one. Still won't feel exactly like one bed, but would eliminate the hump in the middle, i would think.

A cal king is 72" x 84" a twin XL is 38" x 80"(together = 76" x 80"). The frame or bed you have will not fit unless it could be adjusted.