The shelf life of D-23, which contains no added alkali like boxrax, is the same as that for D-76. So mixing up two solutions with the borax in one and the rest of the chemicals in the other doesn't provide any advantage.
Forgive me but I don't see any real problem. Most people can mix up a batch of D-76 in less than 30 minutes. This includes the time to get the balance and chemicals off the shelf and cleanup afterwards. It's not rocket science. Just mix up what you think you will use in 6 months. Be that a liter or a gallon.