Andy, There is a specific protocol for casting 'bi-metal' designs.
It can be quite a job at the craftsperson level, though large casting
houses do it regularly.
I don't know what your time schedule is but, if you have time, try
to obtain a little booklet called, 'Bi-metal, Multi-metal Casting' by
William E. Garrison, pub Magic Circle Corp, 714 10th Ave, E.,
Seattle, WA, 98102, in 1972. Maybe Amazon has it. While somewhat
crudely published, the booklet provides a great deal of information
on this subject at the craftsperson level. I am sure there must be
other tomes out there on the subject...this just happens to be in my
Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry! @coralnut1