Big disclaimer: NOT A RADIOMETRIC DATING EXPERT!
I think it is possible to have other isochrons produced, other than those that determine the age of a rock.
Example: I believe if you sample a number of different igneous rocks that are of different ages, but are of a common magmatic origin, you can produce a isochron. But the isochron will determine the age of when the different magmas fractionated from a parent magma.
I may be wrong - See above disclaimer again.
I now stand by to hear from a real expert.
Moose