It takes approximately 6 weeks for the IRS to process an accurately completed past due tax return.  It can take longer this time of year with the IRS busy processes 2019 returns.

You should mail each year in a separate envelope, make sure you sign your returns.  You can mail them at different intervals or at the same time, just as long as they are in their own envelopes.  See the following from the IRS.

Filing Past Due Tax Returns

 https://www.irs.gov/businesses/small-businesses-self-employed/filing-past-due-tax-returns