If it seems a bit hard treadling when you ply, you might try adjusting the tension (the string over top of the bobbin) a bit.
I use my Lendrum mainly for spinning angora and for plying. I do have the big ol' plying bobbin. I take two FULL bobbins from another wheel and ply them together on the Lendrum. Boy, howdy! Do I get BIG skeins!