Can I mail two quarterly payments together in the same envelope as long as I include two checks and both vouchers?

Yes but one might get missed.  You can send one check for both.

or you can avoid sending a check and voucher completely.  the IRS has direct pay which will allow you to make an electronic payment using Direct Pay.  at the end you get to print a receipt and the IRS even sends an email confirming the withdrawal 

https://www.irs.gov/payments

there are things you have to be careful about

specifying what you are making the payment for -  2019 balance due or 2020 estimates

routing number that the 9 digit number at the lower left of your checks and the bank a/c number 

if you prefer mailing the check and voucher follow the instructions for the info that you must put on the check.