Currently, the "Version" field of our bugs can only be set to one of
"compat (1.0)", "core (2.0)" or "unspecified".
Why not to create a new "both" option to explicitly indicate that the
bug is valid for both versions?
So, "unspecified" would only mean that the bug has not been tested on
both versions.
(Ideally, the "version" field should consist of two checkboxes, one for
each version.)