Separating insulation in half??

I’m insulating my 16x20 workshop. I’m currently doing the roof and want to leave the ceiling open to store things above. I have 2x6 rafters. I left a 1” air space for air to pass from my soffit to the ridge and I used 1.5” foam board that I got a really good deal on. It leaves me with 3” of space. I was wondering if fibreglass insulation can be separated in half. Ex: use 6” insulation and separate it in half to get 3”... I’ve searched everywhere online and can’t find an answer. Any help would be appreciated. Thanks