Hi Tony,
use masking tape on the back taking advantage of the small overlap?
I'm not sure you really want an overlap, the bump will show!
I did this on a smaller scale some time ago and butted, careful trimming is required. Masking tape is too feeble, a reasonably wide strip of card would be better with spray adhesive on it, or something slower at drying possibly.
Place one print on top of the joining strip and then butt the other print to it and leave to set.
Will your printer take a roll, that would be far better!
Terry