If you actually hook the rudder to the rudder channel, yes that trainer will help. After all, thats the only way you could turn so you'll get used to using it very quickly. But as others have said you won't be using it the same way you would with ailerons on other planes.

A better way may be to cut your aileron throw way down on one of your other planes so you have to use the rudder more to maneuver as you normally do. I have a trainer that rolls veeerry slowly and it gives my rudder thumb a good work out as I hardly ever fly straight and level

Slow rolls, knife edge, slips, stall turns, and flat turns also exercise that left hand.

As far as twins go it depends on the plane. On engine out situations you definitely have to have your left hand ready.