cherry grafts done now, have a chance of forming a few blossoms at the bottom of the new growing shoot next season. Especially when grafted on a mature already flowering tree.
your tree’s are not that far apart though. So i doubt grafting a scion of each to the other will yield improvements.
Best to find scionwood of a self pollinating cultivar. One with the mutated S4 allele
from this topic
might be usefull
pages 30 and 31 list a lot of the self fertile cherry’s. Most have a mutated S4. But it seems they have also found/made some with S3’ or S5’
I’m curious how those arose. If they where made by irradiating pollen or where found naturally.