In my experience, cuts are pretty safe in June. If you’re just deciding between two strong leaders on a young tree and cut one to a very generous stub (inches long, say), the worst that might happen is the stub gets some dieback, but you’ll still be making progress towards a future orderly flush cut. I’d strongly prefer doing that cut now than fall (where now = when you posted your last comment, apologies for not spotting this earlier!).
Do you have a laser-sharp positive ID for this one yet? Id still say it’s populus, but not tremuloides (aspen), not trichocarpa (black cottonwood), not nigra (black poplar / lombardy poplar). I have a few dozen cottonwoods and black poplars across a wide range of ages, and can at least say that if you trunk chop a finger-thick 4 foot tall cottonwood to 5 - 12” tall, you can get buds out of the tip of the cut or close to it within a few days. If on the other hand a seeding is left with a strong imbalance of power after a big cut (say you leave a tip running on your favorite trunk candidate but hack back the other one), the big cut may be subject to some recession to a previous node. In other words, populus defends a singular remaining cut tip with all the effort it can muster, but if there are bigger better deals still remaining elsewhere on the tree, then the cut tips will get a “meh” response. So leave generous stubs (as always with pioneer species).