Can I have OS Mojave on a separate partition to OS Catalina?

I have just bought a MacBook Air 2020, which I set up from my previous (early 2014) Air. The Air 2020 is on OS Catalina. I still have one or two 32-bit programs, so I would like to set up a separate partition with OS Mojave on it. I don't want to go the Parallels route; I just want a separate partition. On my 2014-Air, I have two partitions: both partitions with Mojave, but one that has the 32-bit programs so that, when I upgrade the main partition to Catalina, I will still be able to use those programs by booting from the Mojave partition.

Looking at the forums and other advice on the net, most seem to be describing how to instal Catalina alongside Mojave, but I want it the other way round: I have Catalina, and want now to instal Mojave alongside Catalina. Incidentally, I don't at present have an installer for Mojave, but I do have one for El Capitan - which would be fine, if it worked. (When I created the partition on the 2014-Air, I used the El Capitan installer, and then upgraded to Mojave.)