A 2 cents answer to your million dollar question.
If the epoxy are shafting epoxy with the same curing time - quick set or 24 hours . I guess you could mix and use.
I would mix a small portion and let it sit in the room temperature, exam it after the curing time is due, to see if the epoxy set or not.
As for hardening of the resin, they all do that, depending on the condition you store it under, some will last a year and some will last more than a couple of years. If the RESIN had lost it's moisture and become hardened but still soft enough to be scoped up by a screw driver, it's ok to use. Be careful of the portion you use since the resin lost it's solvent it will be more concentrated. Again mix it up well to have the texture close to the newly bought.
I had used hardened resin and no issue there.