Both my boys will be graduating from two different schools and in an invitation I'm inviting everyone to "Ragen's and Connor's graduation". So I show possessive for with both boys and would the word "graduation" be singular or plural??
I would write an invitation to...
... celebrate Ragen's and Connor's graduations
but I think
... celebrate Ragen's and Connor's graduation
is fine too. - The plural makes it clear that both of them have independently graduated.
Also you could, of course, have
.... to celebrate the graduations of Ragen and Connor
.... to celebrate the graduation of Ragen and Connor
with this second way around I think I would go for the singular. '.... to celebrate the graduation of Ragen and Connor'
My experience is that there is never enough room for everyone who wants to attend a graduation ceremony to fit into the space provided, so I assume you are inviting friends to the celebration that you are throwing after the formal events.
So, why not try "Ragen's and Connor's graduation celebration"?