Yes, speaker outs are as far as I am aware always parallel (can't think if a series coupling) but when you have single outs of one type Speakon or 1/4" they don't always follow that rule and it is a case of one or the other. I'm trying to recall which amp I had a couple of years ago that had this and plugging into one output disabled the other; you could use the Speakon or the 1/4" but not both. It makes no difference as you daisy chain from the cab... assuming cab has in/out. There could well be amps with one of each that work in parallel but you'd need to try it to be sure (you'd not do harm as it would be parallel or not work).
I think the caveat is that anyone plugging in cabs needs to be aware of is cab ohmage ratings and being sure to use appropriate speaker cables wired correctly. Not unheard of for noobies to use wimpy guitar leads or homemade leads wired in reverse.