Can I send 2 quarterly estaimted payments in the same envelope?

Sure, you can use the same envelope.  I would, however, include both vouchers and two separate checks for the best results.

Thanks. This is an issue again this year (2020 tax year) for those of us in Texas because of the declared disaster earlier this year. I believe it also applies for Oklahoma and Louisiana. So it's good to know the the April and June estimated taxes can be in one envelope.

I wouldn't chance it.  They might miss one.  If you are sending in 2 estimates at the same time why wouldn't you just write one check for both and send one payment slip?