Sealing gap between double BCI – GreenBuildingAdvisor

Still sealing this rim joist and have run into this gap between double joists, which, unfortunately, I have a few of.   As you see in the photo below there is no access to the cavity between the joists so I think my only option is to drill holes where I can and blindly fill the gap with foam, hoping it air seals the cavity?  Or is there a world in which it is better to leave it alone?  Other ideas? 

Thanks so much, Ian