Place the two sheets of T-111 side by side as they would be installed on the wall, with one to the right and one to the left. Allow the one to the right to overlap the sheet to the left by one to two inches. If you are splicing a new sheet to a sheet that is already on the wall, then temporarily screw the new sheet to the wall, allowing it to overlap the old sheet just as though you were laying the two sheets side by side.
Clamp the sheets together if you have two sheets that are lying side by side with a two-inch overlap. Using C-clamps, clamp the sheets at the seams where they overlap. If the sheets are in place on the wall, then clamping is not necessary. Label each sheet in order to record sheet placement after you have made the splice cuts (T at the top of the sheets and R and L at the seams where the sheets will face that position).
Snap a chalkline down the length of the two sheets where they overlap. If the sheets overlap by two inches, then measure in one inch from one side of the overlap at the top and bottom of the sheets and place a mark. Snap a chalkline connecting these points.
Set a circular saw blade to a depth of 1/2-inch at a 30-degree angle. Cut the sheets completely through at the chalkline. Unclamp or unscrew the sheets and remove the small overlap cut pieces. You can now place the sheets together with perfectly spliced seams that are identical.