Invitations

Ok, my uncle is divorced. He lives alone. He has 2 sons who live with their mother. I want to send my 2 cousins their own invitations but since this is a last minute thing, I don’t have that many invites left over so to make sure I don’t run out I wanted to save by sending only 1 invite addressed to the both of them. My question is, how would I word the invitation? On the outer envelope would I put: Mr. Joe Smith
Mr. John Smith
123 Wall Street
New York, New York 12345

And for the inner envelope, would I write: Mr. Smith and Mr. Smith ?

Or use their first names?: Joe and John Smith

or just: Joe and John

I don’t know...Im so confused. Please help!

Forgot to say that they are both under the age of 18, and I am not inviting their mother.

You can do whatever you want for the inner envelope..depending onhow formal you want it to be.
As for the outter envelope, etiquette says that if they are old enough, they should each receive their own invite... but etiquette schmetiquette... I'm doing the same thing because I don't have enough invites, so 2 of my cousins are going to have to share their invite.