DS Birthday is 11 days before Xmas, and I just mentioned to my family that it's important to me to separate birthday and christmas. One day is all his, the other day is for the whole family.
I keep the Xmas decs in the box until the saturday before Christmas. Of course we do the advent calendar. Tho this year I'm intending to make my own, so I can incorporate his birthday into it. So he can see that there are 2 special days in December. He'll be 4 this year, so is beginning to understand. Last year he got to the first week of the New year, took himself off to his room, sat down and said, can we open some more presents now... and i think he asked the day after, if we were going to have any more presents..
I'm going to make Easter a little more special though, so he doesn't have all year to wait then it all come at once. I'll get him his summer clothes for presents for example.
So my advice to OP, don't be too mercenary and tell people, I don't want you combining gifts, unless they bring it up. If they do, just say, no, I would like to keep the 2 days separate and special.
In this particular instance, When any hint of a gift conversation comes up, seize your chance and point out, gently, that your DC got an all in one christmas, birthday and brought back from holiday gift and the other child got separate presents. State that it's perhaps not an issue at this age, but it will be once they are able to talk and compare. Try and get them to nip it in the bud sooner rather than later.