I only have one boy. So can't comment from experience. But I agree with Julz.
I think its important they understand that a birthday is about the birthday boy/girl.
I think by giving both gifts can confuse them when they attend other parties, they might expect gifts then too.
I think we sometimes forget how much a child can understand at such a young age. We just need patience to explain things in a easy way for them.
However what I would do is let the sibling help with the party arrangements for the other sibling so they feel involved on the right side if you know what I mean. Like let them choose the cups and paper plates, and help with what goes into the party packs. And let the sibling give all the presents to the birthday sibling and help with opening them. That way they still feel involved and get recognition for helping so still feel important. Visa versa when the other sibling has a birthday
Hope that helps a little