On FB, we were discussing whether or not to do character sheets/character questionnaires. I don't do this, because the one time I did for a class, I lost all motivation and interest in writing about the character. I would be the person who put all those details in instead of only the ones that were needed. I do keep a file with significant information, such as hair/eye color, occupation, and that sort of thing for each character. I sometimes need to know who knows Morse code or whatever without searching the file.
But once I realized I was writing two mystery series, both with thirtyish female amateur sleuths, I opened a file called "Jac vs Ari." Jacquidon's book is in intimate third person; Ari's story is first person. Jacquidon is the college grad who had such great career opportunities, Ari the loner who had a distant mother and an admired elder sister who got into trouble young. Jacquidon is the elder sister to Chantal; Ari is the younger sister to Zoe. Zoe had a child who died last year; Chantal is single and has a boyfriend who never comes on the scene in person but is a comic relief figure who has often JUST called and is often invoked or quoted or has given Chantal some piece of the puzzle somehow (or sent just the right tool to use to fix something)--this is used for comic effect as well as to advance the plot. He's like Mrs. Columbo--remember, we never saw her int he original series, but he was always saying, "MY wife--she thought of this, and I wanted to ask...."
That kind of thing. But the reason for that file was so that I could show others that they weren't at all alike and that I couldn't possibly "just make both books about the same sleuth" for the sake of having one series. The books are rooted in the world known to each character and what happens/plausibly occurs to her.
Jac's books are light, funny, witty, Snoop Sisters-type, like the Anne George novels crossed with Joan Hess (or so I fancy). Ari's stories are darker, deeper in a sense, have more emotional development and change (at the end of the first book, Ari's sister has come out of her self-imposed hermit state somewhat in the process of solving the crime, and may come back out into the world from which she retreated when her son passed.) Jac's story clues use technology/computers. Ari's are more traditional mystery clues.
So now you know. I could NOT "combine" them, no matter if that was a Penguin editor saying so. Note that Penguin has discontinued most of their cozy mystery series just a couple of weeks ago. They no longer have that hashtag on Twitter. SO they're serious about it.