I made some with 3 oz Polarguard 3D, using a pattern i Already had developed for shell pants. I found I had to add substantial extra girth to the outside shell pieces to allow for the insulation to loft - so if you are working from a shell pants pattern, you'll end up doing the same.
As to splitting the insulation, from my experience you'd have a heck of a time getting it to split evenly or anything close to evenly - meaning you'd end up with a large variation in thickness. I'd look for some insulation closer to the thickness you are after.
On my pants - and a jacket I made at the same time - I sewed the insulation to one layer of the fabric, around the edges, then sewed the pieces together into pants, and then sewed the other layer of fabric to that, so the insulation is stabilized at the edges only, and not entirely sewn thru at the crotch seams or leg seams.
I found that pinning the insulation to the fabric helped a lot - without the pins I could not keep the insulation lined up with the fabric and got into trouble quickly. Hte other issue was the insulation getting caught on the presser foot. The only fix I found for that was constant vigilance. If I could have a large, bowl-shaped presser foot that would be great but I haven't seen one.