I silver brazed using Johnson Matthey easy flo no 1 solder on
hundreds of brass joints, with not one failure, specially as the
joints were made with the 2 parts flat, then bent afterwards to make
a C shaped cuff bracelet. Needless to say it was then silver plated.
Also one design was also in 3 metals, however the base was not
silver but stainless steel, then brass, then copper.
You dont say what the mixed metal piece is? can you substitute
stainless for the silver, tho you will neded a flux specifically for
s/steel you will get it at a welding supply store, and you can use
it on all your soldering jobs.
It has aan additional use, not normally known about, it dissolves
fire stain out of sterling, you need to continue applying the flux
powder with your s/steel rod so it runs off the piece taking the
copper oxide with it.