Talk to the place you purchased the insulation and ask them.

Thank you Robin and David. I somehow missed the notifications for this post. It does seem like a lot of trouble. We are going to try to use some if up in the garage.

Blown-in will settle over the years, and will lose R-value. Fiberglass batts will not. Everyone does blown-in in their open attics because of diminishing return - it's simply cheaper to do it that way, you can easily pile up as much as you want, then 15 years later you can just blow more on if you want.

Roll out those batts in the open cavities then toss the blown-in over it, nothing to lose really

If you think you need a lot more, you can rent the machines from "big box stores" and buy bags of that to blow in yourself too

Are there not some disadvantanges to using batts in an attic in terms of filling in all the gaps?

Not really, if the batts are sized properly. Could say the same thing about walls, not every stud is 16 on-center so it's only as good as the insulater field-cutting and stuffing glass in the cavities. If you wanted it perfect you could spray foam everything, but again that's $$$ aka diminishing return on investment.