If your are talking about making one big piece out of two smaller pieces of fleece, you can butt the pieces together and zigzag across the blanket. I did this once to make a blanket for my bed and didn't care that there was a line of stitching across the middle as I knew it won't be seen. My blanket has held up for well over 10 years now.
If you are concerned about the look of the seam you could add some appliques on the seam.