Custom twin-lead is easier to build than coax... at least that's my opinion. Still, either can be built by the experimenter. Wikipedia articles provide sufficient math and related theory to get one started. And a quarter wave matching transformer can be built from either twin-lead or coax. Given the integrated strip-line balun built into the 91XG, a coax based matching system would be the logical option. (I've built 'ladder-line' style twin-lead using bare copper wire and spacers made from plastic pail lid material. One could use aluminum or copper tube and wire to build short sections of coax... the dimensions are very important.
http://en.wikipedia.org/wiki/Quarter...ce_transformer
http://en.wikipedia.org/wiki/Characteristic_impedance
http://en.wikipedia.org/wiki/Twin-lead
http://en.wikipedia.org/wiki/Coaxial_cable
The 'cost' of a quarter-wave transformer is that it will have a relatively narrow operating bandwidth compared to a wire-wound broadband transformer. A quarter wave matching system designed to operate in the UHF band is going to be very inefficient in the VHF band... but your proposed application would not be affected by this given the 91XG is also limited to the UHF band already. (I toss this in for anyone hoping to try this with a pair of all-channel antennas.)
I crunched a few numbers and came up with a 'home brew' coax with an impedance of about 103Ω. I used the ID of 3/4" type M copper pipe, 0.811" and the diameter of #24 AWG wire, 0.0201". Presuming air dielectric I ran the numbers using the calculator at
http://www.rfcables.org/coax-calculator.html.
The #24 AWG wire is commonly used in CAT-3 through CAT-6 type phone and data cable and the Type M pipe is the common thin wall copper pipe used for plumbing. If you use an RG-6 cable, 1/2 wavelength long (electrical length) to connect each antenna to a custom built 106Ω (nominal) coax section that is 1/4 wave length long (electrical length), you would be in the ball park...
http://en.wikipedia.org/wiki/Copper_tubing
http://en.wikipedia.org/wiki/American_wire_gauge
http://en.wikipedia.org/wiki/Wave_propagation_speed