How do you take 2 pieces of fleece and put them together without showing a seam

You could make a french seam.

I would take scraps & experiment. Do you need it not to show from both sides?. Try overlapping a little with selvage on each side or just take a tiny seam allowance & see how it looks. No way it's going to look seamless but you might minimize the seam's appearance

Hard to do. You can cut edges and tie the two pieces to make a No Sew blanket