That's a problem then. Although they do sell, somewhere, corrugated "nails". For just this purpose to join boards side by side. You probably know what I mean. They're almost an inch long, wavy, have points at the bottom of each wave, and the grooves that make up the wave lean towards the center. Those on the left lean towards the right and vice versa, so that when you hammer them in, they pull the two boards towards each other. 3 or 4 of them would be enough and they can go not at the very end, but where they won't be seen. If they can be set in, a heavy coat of the same paint used for the wood may hide them completely.
Better than those plates with 30 or 100 nail-like projection bent out of the plate, like are used to put pre-made trusses together.