How to populate a Backbone model from multiple URLs

I would like to use a single Backbone Model that is composed of data from different URL endpoints. Is it possible to specify multiple URLs within a single model? I'd like to avoid calling AJAX by hand.

Until now, I've been using jQuery Deferreds to call multiple URLs and synthesize their results into a single object.

I can think of two options so far: building a Backbone model with a partial model for each URL, or using one URL and then overriding Model.fetch() to call the other URLs. But neither one makes me comfortable.

(Or I can try to bribe the API developer to provide a merged URL endpoint...)