Two strands of sport is supposed to make worsted, but it may come out a little thicker since they're just held side by side instead of plied together.
explains the ply system equivalencies which is probably easier to go by -
2ply = lace
2 2ply = 4 ply = fingering
1 each 2ply + 4ply = 6 ply = sport (more or less)
2 4ply = 8 ply = dk
2 sport = 10 ply = worsted
2 6ply = 12 ply = aran (heavier worsted)
2 8 ply = bulky
anything more than that is super bulky or super super bulky.