Do a quick search on here, I'm sure it's been sked about before and you aren't the first one to need to do it. Just going from memory, I don't think there are any real problems with compatibility between the two. Especially if you are using the same thinner on both, which I think would be key here. Put a toot of the SIg clear on a scrap piece of balsa, let it dry over night, sand it a bit to take the fuzz off, then a toot of the Brodak dope. Let the SIG dope dry on the model as long as possible to gas off as much as possible.
Type at you later,
Dan McEntee