We all talk about safety when installing multiple drives and operating systems.
Along those lines, keep in mind that when you install the second drive and go to install PCLOS on in, it may show as drive one depending on the hardware.
If you are not paying really close attention, you could see that and tell the installer to install PCLOS to what is now drive two and unintentionally overwrite your existing installs.
If your drives are both in externally accessible bays, consider removing your existing drive before installing PCLOS on the new one.
That way you are guaranteed that you will not mess up your existing installs.
Also, it will guarantee that during the install and setup, it will be the one that boots so you will be able to do the entire install and setup without any confusing surprises.
Also, some may disagree, but when using an old drive for the second drive, I always create the partitions for the new install using gparted and then tell the installer to use existing partitions.
Otherwise, you frequently get the unexpected message to reboot the system so that the partitions can be created etc. I personally find this to be an annoying aspect of our installer that is no doubt carried over due to its heritage.