I use a dripper and just drip in new flavor* to try and let it mix. Then when the recipe's "complete" I do the exact same thing so it's all good. I pretty much never vape a "pure" flavor, it's always mixing on the fly, no "flavor" can compete with that goodness. I usually juice up a new wick with caramel or marshmallow followed by a tobacco no matter what I'm going to be vaping. Also, I really like doing tobaccos (incl. NETs) that are sort of pre-diluted, perfect to use with other flavors but not exciting by themselves. I find some flavors very noticable at the most subtle levels when they're cutting across another flavor. It's chaos but eventually works.
*to be clear here, this is a ready to vape ejuice. Another advantage of mixing up single flavors is to go on and get them steeping, some of my combos I first time mix from juices into a bottle, some I just get experience in the dripper and then do combos straight from flavorings and base, or even both, esp. when something's really strong and can use 2 levels of dilution.