M-Query previous month filter is not working like SQL Database for Previous 2 months

Use Date.AddMonths. See below:

Date.AddMonths([Date], -2)

That will move the date exactly 2 months back where possible. Notice how I moved Oct 31 1 month ago. There's no September 31, so he moved it to September 30 in that case.

To use it as a filter using today's date, you can use this:

= Table.SelectRows(#"Added Custom1", each [Two Months back] >= Date.AddMonths(DateTime.Date(DateTime.LocalNow()), -2))

That will make it all August 15, 2020 and later.