Parallel means you’re connecting the two packs to gain a higher capacity at the same voltage.
Series means that you’re connecting the two packs to increase voltage.
Connecting random packs in series is considered a big no-no for many reasons and it’s generally something that we will all just tell you not to do.
Connecting packs in parallel is a much different story and it can definitely be done successfully but it should definitely not be treated as a set and forget type of thing, there are caveats.
This thread is a great place to gain some insight.