During the trip to Northern Brewer, I think I finally got the right parts to resolve my CO2 system issues. I brought my situation up with the store staff and they pointed out that there is an easier solution to the fitting problem than the one I postulated in my earlier post.
As it turns out, the 1/4" male flare fitting on the primary regulator is part of a standard assembly that they add to the regulator bodies they get from people like MicroMatic. The regulator body itself is threaded and can accept different fittings. After I described my situation, the NB guy (whose name I didn't get, more's the pity, as he was really helpful) said "well, you don't have to fool around with inline splices or anything, just unscrew the 3/8" barbs on your secondary and replace them with THESE" - whereupon he opened a parts drawer with a flourish and produced some of the flare assemblies like the one in my primary regulator.
We talked through the scenario I was building and I ended up with three of the valve/fitting assemblies and a brass "Y" adapter that will screw into one of the secondary regulator openings after I remove the barb and provide a mount for two of the flare fittings, while the other one goes directly into the secondary.
So now, the plan is as follows:
(1) remove the barb fittings from the outlets on the secondary and replace them with (1a) a valve/flare fitting and (1b) the "Y" adapter with two valve/flare fittings;
(2) remove the valve/flare fitting from the primary outlet and replace it with one of the barb fittings removed in step (1);
(3) connect the primary outlet to the secondary inlet with 3/8" ID tubing;
(4) connect the kegs to the secondary outlets with the preassembled lines
It's all so simple when you talk to someone who knows what they're doing!