Will there be a problem if I mix the two metals on an item?
IMHO no, certainly not. Your Argentium Silver (AS) will behave more
or less like AS and your standard sterling (SS) will behave pretty
much as you'd expect it to. You may get a little cross-contamination
during soldering and polishing processes but you'd expect that: do
what you can to minimize it and you should be good to go.
Recently I heard that a local supplier will not carry Argentium
because he says the two cannot be stored together.
I know of no good reason why this would be so. For instance I've
been wearing a AS/SS loop-in-loop bracelet for several months now and
can see nothing in it that would support said supplier's claim. On
the other hand maybe what he really meant was "I don't WANT to carry
the two of them" in which case his statement makes a little more
sense although it would seem to indicate that he has an honesty
in The City of Light