I see there are quite a few languages that aren't available for English speakers, like Catalan. However it's available for Spanish speakers.
Do some of you Duo users learn one language to learn another one? Is it feasible?
What you're describing is called "laddering". (Though, as a bunny myself, I quite like that you've called it "hopping" :D)
There has been good feedback over the years here in the forums from people who are laddering. It lets them learn one language, then start learning another without completely shifting their focus from the one they were just working on. ^_^
Yes. This is how I started duolingo. When I started I spoke a pretty proficient level of Spanish, and I decided to learn french from Spanish. It was tough, because I often ran into Spanish words I didn't know in english, but I ended up learning them and their french translation in and of themselves. I got frustrated by french and eventually found that portuguese (from spanish) was really, really fun, and easy (Doubt you know about of dopamine's workings but it has to do with this). I have spent a ton of time doing the portuguese course for spanish speakers, and even completing it. I have not taken on one language seriously from english and its interesting, and a fun trip. I did a bit of the spanish course from portuguese to try to think from a different angle in the language and that has been helpful. Even further, I am I think level 8 in French from Portuguese, which I think is a ton of fun, yet hard, and its possible because of the investment I have put in—but not just really even with duolingo.
Something kind of funny is that although my native language is english, Its pretty hard for me to change between portuguese and English (because I have learned it all from spanish, and when I speak in portuguese, its basically like an off kind of brand of spanish in a way; I use similar brain pathways and my understanding of spanish grammar to suceed). This week I tried to do the tests for the portuguese from english tests and it was hard! There were things I knew how to say in spanish and portuguese, but the english translation I was often at a loss for; it took me a while to complete it.
It is totally possible, but know that I have a lot of linguistic knowledge and time spent into learning languages outside of duolingo: I am fluent in Spanish, gone through all the grammar, I have read a lot in the language, I have a pretty good basic understanding of arabic (and fun fact, all of my note cards for learning the language are from spanish and half of my notes are in spanish) —which is complicated, as one can image—and I have functional portuguese; these facts all help me with the whole laddering and mixing concept. I have found it easier to learn other languages starting from Spanish because it works for me and I am good at it.