Yes, you probably do want the cables to be the same length electrically, especially if you don't have an automatic way to apply a calibration correction, just for convenience if nothing else. 4 degrees at 1300MHz is about 1.5mm, assuming solid polyethylene dielectric. You may be able to achieve that sort of matching if you are really careful in your construction. What sort of connectors are you using? If they are they type where you solder or crimp the pin to the center conductor and put the line into the body of the connector until the dielectric hits the dielectric of the connector, you should be able to come close to that sort of match in physical length by very accurately matching the length of the dielectric, and cutting it very cleanly perpendicular to the axis of the coax. If you're using good coax, the variation in velocity factor from piece to piece should be small, and so as long as you don't try to make really long cables, you should do well.
Note that you can calibrate out lead length differences. A difference leads presumably to a phase error that ramps linearly with frequency. Does your VNA have a way to remove/add a time or at least a constant phase to the readout? And you can perhaps arrange to measure the electrical length differences. If you can do that, then you can make up, say, four of the 24" (nom) cables, and pick two matching ones to go to the two receiver inputs. Note that the length from the source to the coupler (T/R test set) is NOT CRITICAL at all. So make some cables as close to the same as you can, and pick the best-matched two for the receiver cables. That should let you make good reflection measurements. Then you can either adjust the length of the longer cable, or calibrate out its length error, for transmission measurements, assuming you care about phase. Often you'd have to account for the length of connectors and other cables to your DUT anyway.
I've made some measurements on some rather expensive test cables and found the electrical length to vary more than I'd have expected, though they weren't matched sets. I'd hate to have to select a matched set from such cables, though, as it would take quite a few cables to assure that I'd get two that matched close enough! For several hundred dollars per cable, I'd have expected they'd hold them closer to the nominal length.
Cheers, Tom
(Note that some extra care is taken in the design and construction of the coupler to get accurately matched paths to the two receiver outputs!)