Since you seem to have particular people in mind for this, you might just mention it to them over the phone.
If you are having called-in RSVPs, then you could say "BTW, just so you waon't be surprisd - we are doung a joint party, but I didn't want you to feel obligated" etc.
If people aren't calling in their RSVPs, then you might just call them up and say, "Some people have been concerned about this, but I wanted to let you know that bringing a gift to both parties is not expected".
And then some people are just going to do it, anyway, and there's no way of talking them out of it!