How can I configure Ubuntu to offer X logins on more than one virtual terminal after booting?

I have long wanted to do this and been stumped as to how.

I found a tantalising hint here:

https://www.maketecheasier.com/run-multiple-x-sessions-without-virtualization/

This is someone wanting to do same. Alas it doesn't work (I tried ;-), which is by the by as it's also subtly less than what I want.

I want to configure my system so that at boot it offers an X login window on more than one virtual terminal.

If ttyn is the virtual terminal activated by pressing CTRL+ALT+Fn then:

For example, currently tty7 has a login window and when I log in I have X on display :0

I would like to configure my system to offer a login window also on tty8 and be able to log in and find I'm on X display :1.

And so on. I mean two meets my needs for now. But this should easily generalise.

I am surprised at how hard this is to be honest and that I've not found more evidence of people wanting it, as it seems desirable for so many different reasons to me. The only con I can see is memory consumption, but seriously I'm on a box with 32Gb of RAM. I can't see an idle login window eating a serious chunk of my RAM. But what I want to do right now for example is logon as two different people and flick back and forth comparing things as I do some diagnoses. But in general I find it desirable to be able to run different sessions in parallel on different ttys and the technology is so clearly there. It is the savvy I lack.

Things I have tried:

1. If I "switch user", then I'm offered a new login and that is on tty8. I can get back to my first session on tty7 by pressing CTRL+ALT+F7 and the new one by pressing CTRL+ALT+F8. What I'd like to know is how to configure a system so that tty8 offers a login prompt after boot without my having to do that (i.e. Switch User). For a number of reasons, though learning and understanding the X startup system and how it ties in with virtual terminals (ttys) and is configured is a big part of it.

2. If I got to tty1 and log in and run startx it totally locks up my tty7 session. Only way I can find to get it back is to reboot.

I'm on a Mint 19 box which is a derivative of Ubuntu 18.04. But I've wanted to learn how to do this for a long time I admit. What system files define where and when X login screens are presented (by default clearly on tty7 some time after boot) and then tty7 is made active. I'd love to know where that is configured and how to tweak it to throw another login screen up on tty8, and or more ttys and how to tell it which tty is visible after the boot sequence is finished.