Do any plant galls require more than one host?
Yes. Requiring multiple hosts is common for galls created by rust fungi, of which there are several thousand species. Their life cycles can be complex with several distinct stages, and they rely on infecting plant hosts at different stages. They also tend to be very selective as to their host plants.
Gymnosporangium clavariiforme is one such rust, infecting juniper and hawthorn. In spring, it produces orange tentacle-like structures on juniper (which don’t harm the plant). In summer, these produce spores that go on to infect hawthorn, causing a yellowish scarring on the leaves and galls on the leaves and fruits. It’s these galls that release spores which go on to infect junipers, so restarting the cycle. Infected hawthorn can and do experience loss in haw production.