Another Riddle

[Dragonboy]

1,2 cross; 2 returns (4 mins so far)
5,10 cross; 1 returns (15 mins so far)
2,1 cross (17 mins total)

You are never again allowed to participate in riddles
I would have loved to see the others try at it for a little longer.
Another combination could be:
1,2 cross - 1 returns (3min.)
5,10 cross - 2 returns (15 min.)
1,2 cross - (17 min. total.)