could I selectively thin the suckers as both plants spread and grow to maturity

I much prefer Syringa vulgaris with multiple comparatively young, vigorous and vertical stems resulting from a continuous procession of new ones being kept coming from the base. Rather than limbed up into a tree shape, with the specimen consisting entirely of old wood. And becoming twiggy and leaning.

Yes, I feel the same way, and this is how they would be maintained. New stems allowed to mature and the oldest removed occasionally.

"Selective thinning" in my original post referred to removing (old) stems as the two plants matured in such a way as to promote an intermingling of the two plants rather than a "purple on the left, white on the right" appearance.

But is there any worry about the long term health of two plants "right on top of each other" like that?

Or are the two plants likely to compete to the point that one of them loses out

==>> i doubt it ... presuming they are the same size at planting ...

just do it.. whats to lose ... ken

Well, one lilac was free, and the other was a $5 end of year close out.. so not much to lose :)

when you have an inspired idea.. cost is not an issue ... especially when the cost is probably less than a cup of some foo foo coffee .. lol

ken