Yes, slip the last st that you picked up over to the other needle and purl with the 1st st on the left needle that's from the other base triangle. Then the next purl row, slip the first st, p6, and purl the next st with a stitch that's from the other triangle. That way you're joining them together. You won't run out of sts because you're taking them off the next triangle. The `dec' sts really aren't decs, they're what joins the blocks/triangles together. Don't overthink it, just do each row as written and it'll turn out right.
sue- knitting heretic