The ports on an R134a system are designed so that only the correct fitting will connect to each one (they are different sizes). Thus, you can ONLY connect a fill fitting to the low side.
BE CAREFUL when adding more refrigerant. Unless you are experienced working on AC systems, it's possible to OVERFILL the system and DAMAGE the compressor. (Just a friendly warning---every year, some guy on one of the Jeep forums manages to do this because he/she doesn't understand the intricacies of the AC system. Don't become one of those guys.)
1992 XJ 2" lift -- RIP
2000 XJ 3" lift -- Australian Ford Big Brakes up front, rock rails, ZJ rear discs (with WORKING parking brake), C4x4 rear bumper/tire carrier, Hella H4 headlights, IPF driving lights, OEM skid plates, OME springs/shocks, JKS LCA's, JKS Sway Bar links.