Making a king sheet out of 2 Twins?

Hi all! I am new to sewing and this forum. I recently purchased a king adjustable bed which is actually 2 XL Twins. At first I thought we could just make the beds like twins but find we are so tangled up in sheets and blankets that we can't sleep. So.... I thought maybe I could make the bottom in twins (since we adjust at different levels) and sew the top twin sheets together to make a king. My question is: How do I do this and make a comfortable seam? I have read about french seams - is this the way to go? Thanks for your help.

That's a good idea. My mom just serges hers. She puts the serged seam upward (away from the body) and it's under the blankets, so you don't see it.