You really don't want to do it that way. For redundancy, you'd use two receiver, each controlling one side of the airplane, and each having its own battery pack. Using two battery packs would be to provide power redundancy to a single receiver. What you're proposing is to connect two battery packs to a single point, and then providing power to two receivers from that point. You do that and you've eliminated the reason for two receivers or two battery packs.

However to do what you want, use a "Y" harness to bring both battery packs to a single source. Make a reverse "Y" harness (one is not manufactured for this) to split the now single battery lead into the two receivers.