W8.1/W10 dual use, same product key?

On my desktop I have a number of removable storage racks in which I can
slip disks in and out in cold swap fashion. This arrangement makes it easy for
me to use different versions of operating systems without having to deal with
the complexity of virtual machines.

Im running W8.1 Pro 64 bit on a SSD as my main production system. This SSD
hosts only the operating system and installed programs. All my data resides
elsewhere. I have cloned this SSD to another SSD which I plan to use to upgrade
W8.1 to W10 when W10 reaches my part of the world. Since I havent taken part in
the Insider Program, I need a few months to ascertain if W10 is worth using as
my main production system. During that time I want to continue using W8.1 as my
main production system.

My question is, will W8.1 still work after I have used its product key to
upgrade to W10? I guess W8.1 will still start but will it be able to receive
security updates after the upgrade to W10?

Many thanks for any insight.

No, completely illegal. Once you use the 8.1 key as the base for an upgrade to 10 you cannot legally continue to use that 8.1. If you decide to get rid of 10 then you can use that 8.1 again.

You can't use them at the same time.

Thanks Ztruker for the clarification. Of course, I do not intend to run W8.1 in parallel with W10 forever. Only a couple of months while Im checking out W10. I will eventually abandon either 8.1 or 10. How long will I be allowed to run them in parallel, if at all?

Not all all. You are not "legally" allowed to run 8.1 at all once you upgrade it to 10.
I don't think anyone knows what will happen if you do what you describe and it's not something that we can discuss here as it's very much against the TOU for this web site.