I clearly cannot guarantee that my series will be popular or well received by readers. I was wondering if it would be beneficial to have another series which is told differently, has a different plot and characters, etc ready in case the first series is not popular.
I would recommend working on Series B, Book 1 while awaiting the query/publishing phase of Series A, Book 1, before working on Series A, Book 2.
However, this is for traditional publishing with the intent on being a fiction writer as a career. A traditional publication can take well over a year to hit the shelves, and it is unlikely an unheralded author will get a multibook deal. The exception is some agents appreciate knowing you are committed to Series A and will consider it a boon to have Series A, Book 2 in queue.
With self-publishing, it doesn't really matter. You can publish whatever you want knowing you can always publish more. Consider, however, if you self-publish Series A, Book 1, it is unlikely a traditional publisher will publish anything from Series A thereafter (with some notable exceptions).
It also depends on your definition of series. Do you mean a single story told in discernible segments, or a group of stand-alone novels with a common theme? The former case should really wait in the traditional publishing framework with an extended outline for an agent or publisher. However, with the the latter you can take more chances. Piers Anthony wrote 4 or 5 Xanth novels before A Spell for Chameleon got published. He was a bit green, and you could basically see where he made impromptu changes in later works as earlier ones were modified at the behest of the publisher.
For myself, I have Book 1s of two stand-alone series that I would love to retire with if either gets any recognition. And I'm now working on something totally different.