Install two different firmware version in same router wrt3200acm

HI, I have to upgrade my Linksys wrt3200ACM firmware openwrt v18 to the latest one.
I know this router should have two partition where I can install two firmware. So I'd like not to perform an upgrade of the existing one, but install directly the new version mantaining the old one, is it possible, and how? I've read that a new installation automatically will be installed in the second partition and the boot switched automatically ... mhh too good to go?!

Yes, both OpenWrt and the Linksys OEM firmware always flash the other partition with the new firmware, and keep the current one as the safe "known-to-work" fallback firmware. If you just flash from the current OpenWrt, the old firmware and the old settings will remain there for you.

Note that lots of configuration has changed since 18.06. You should flash sysupgrade without keeping settings and not restore old settings backup afterwards. Just reconfigure everything manually.

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So, in flash operation menu -> Flash new firmware image, without keep settings ticked, I will load the new img file? I mean, it's the same menu where I could load only the upgrade (bin) image too?
Or ... is there a command line to perform a new installation of the new firmware in the other partition?

Just a normal sysupgrade. With the sysupgrade image.
Just make sure that you disable the "keep settings".

(Even the OpenWrt major version upgrades are normally done with the sysupgrade image. The factory image is only needed for installation from the OEM firmware and some exceptional cases like kernel partition size changes.)