1 pic = 1000 words

How do you plan to "prune back the taller trunk"? This sounds like you are potentially considering topping your tree! Smoke trees (Cotinus coggygria and Cotinus obovatus) tend to naturally have a growth habit similar to what you describe. They gain girth with age. Improper pruning may make the situation worse, but I can't say for sure whether what you are planning to do is the right or wrong action to take, at least without a picture.

google rejuvenation pruning of shrubs.. and do it right..

topping a thin branch.. is going to give you explosive growth at height.. and the thing will fall over ...

imagine if this years growth.. was at 4 feet.. how in the world is that thin trunk going to support it ... it wont ...

from the facts.. i cant tell how long ago it was transplanted ....

its a run it over with the truck plant.. when established.. you could probably cut the thing to 3 inches.. and let it go nuts next year.. and then you will have a more structurally sound plant.. after selectively pruning out some of the duplicate branches ...

but to be sure.. we need a picture.. or two.. to give any kind of specific info ..

ken

Rejuvenation pruning will cause a massive amount of vigorous suckers to form. Removing all but one trunk/sucker will be an ongoing choir and result in an even lankier trunk than before. Since rober49 is wanting a tree form, rather than a shrub forum, I believe cutting the plant back hard (rejuvenation pruning) would result in an undesirable outcome. If a shrub form is acceptable, rejuvenation pruning may well be the way to go.