Yep, tazzle's right, IMHO, if what you want to do is replicate mixing the two together at equal volumes. I would say that even if the result of mixing two juices together was very good, you might still want to experiment with the percentages a bit, because there are interactions between flavors that won't have been present in either of the original juices.
For instance, if you had two flavors that share main molecules, or have very similar main molecules, using half of each is probably going to be best. But flavors that are present in one, and not in the other, where there is no similarity might be a bit weak in the end result, depending on what you're looking for. How much flavoring you want in a
juice isn't just about the strength of the concentrates used, it's about how they interact.
So even using flavorings that are good at a pretty consistent percentage, one recipe might be over-flavored at 8% total flavoring, while another might be great at 12%.
There are some other interactions you have to be aware of, especially as things steep. For instance, some flavors contain a lot of Ethyl Maltol, which can help smooth out harsher flavors, but can also mute some flavors pretty badly, especially over time. If you have a flavor in Recipe 1, which contains no EM, and you then cut that in half _and_ mix it with Recipe 2 that contains a lot of EM it might wind up just disappearing after a little steep time.
There's no way to know for sure what flavors will do with each other, other than trying things out and seeing, but the more you know about your individual flavors and how they interact with other flavors the easier it gets to guess at how things need to be tweaked when they inevitably don't do quite what you expected. This is a pretty deep rabbit-hole, and it can be frustrating, but it is also what makes mixing so interesting, IMHO.