You'd have to test it, to be sure.

I don't think diminishing returns is going to kick in, if they land at the same time.
But don't really know on that.

If you want, adjust it so that only one casts at a time, instead of two.
So twice as many steps, with only one toon going at a time.

In theory, if the target is slowed enough that you're on them, they're dead.
And you're not going to need to slow them again.

Part of the issue is the level disparity.

If you can level, in an environment without that disparity... once you reach maximum level, they won't have that in their favor.
You can do the dungeon route, but that's going to be five toons at a time.
Possibly the raid route, for an older raid... without the raid experience penalty, trash for a level 85 raid might be decent experience... not really sure on that.
PvP is an option, although you'll run into some who are up to 89, in the 85-89 bracket... but no 90s, until you're 90th too.