You need to know your caliper piston OD to determine what MC piston size you need. Too big an MC, and you won't produce enough system pressure to stop properly. 2-piston calipers don't necessarily mean larger total piston area than a single piston.
I've come up with these examples of single and dual piston calipers, including what size MC the OEM used:
Dual piston fronts with 1.181 OD have an area of 5.14 sq in and were used in an '03 Explorer, with an 1-1/16 MC.
The dual piston calipers with 2.18 OD pistons have an area of 7.46 sq in, may use an MC larger than 1-1/16, but maybe not.
A 2.934 single piston caliper off a 1/2-ton '79 Chevy has 6.8 sq in, and used a 1-1/8 MC; the 3/4-ton '79 Chevy with 3.14" caliper piston has an area 7.7 sq in and used an MC with a 1-1/4 piston.
Remember, the trucks these calipers/MC's came out of generally have much larger brake boosters than will fit in an XJ, or use hydro boost. So, while you may be able to bolt in the 1-1/4 MC, you probably won't be able to produce adequate pressure in the system. If you install calipers that require MC's 1-1/4, or larger, due to required volume increases, you may need to use hydro boost instead of vacuum boost.
I'm using the '79 Chevy 1/2-ton brakes along with a 1-1/8 Durango MC bolted to a late model XJ booster. My pedal is firm, and requires a little effort to lock-em up; could be I'd be better off with a 1-1/16 MC.
