Hrainian, if you're doing it by yourself, the best way I find, is yes, to camp one player in a safe spot, use the other one to do the level.
However, each player has his own lives until you get this item twice, and if one of them loses all their lives, it's game over regardless.
So you want to alternate them, to make sure they both have enough lives to get to 49. Also, remembering the layouts of the levels is important, though I'm sure you do.
Lastly, if one of them goes to the bottom at the end of the level, both of them, HOWEVER...at the next level, the one that went down will show up first, then the next one will show up a few seconds later, it's not simultaneous...that's what makes this a tad difficult and requires you to have the levels memorized and where to fall to be safe.