Our house came with a baby swing in the garden. As you can see this has been brilliant and well used by both children. Now though, they are both too big for it and it's a little redundant.
So yesterday I took down the swing, and inserted in its place a bike tyre that had been hanging unused in the bike shed. Now they have a swing that they can both use without any help from me! Result! It's not perfect, but it's definitely better than a swing that neither of them can use.