Many ornamental cherries are grown on a hardier root stock, that is, they are propagated by grafting onto hardy wild cherry saplings. This is because the ornamental variety will not produce offspring that are true to the parents, if they produce offspring at all.
All are grafted plants and mainly on wild cherry (gean) rootstock. Such trees should be planted with the graft union above ground level. Seed, rarely produced, will give rise to new hybrids.
This is a picture of an ornamental cherry of the pompom variety:
My guess is that the tree in your picture sent a shoot off the main trunk under the graft site, resulting in a branch of native cherry, which is mostly white and a much simpler flower than the ornamental variety that was grafter onto it. Another possibility is that two different specimens were grafted onto the same stock (but since the white blossoms are very simple - like a wild cherry - I would guess the former occurred.)
This is a picture of a wild cherry (gean):
So you have the grafted ornamental cherry blooming alongside of the rootstock cherry it was grafted to.
The rootstock (although usually wild cherry) can vary, so... can't identify with certainty the rootstock, but can explain what you're seeing.
It's not rare, but it's not usually desirable. Some newer growers are grafting several varietues of cherries onto the same rootstock to intentionally produce a tree with different kinds and colors of flowers. See, for example, this artist's work.
The Hamada Cherries