Practicing a language from the perspective of your target language
I decided to start a new German tree, except from the perspective of an Italian speaker. Let me just say...wow, that was pretty cool. I don't think I'll get too far because I'll hit a point where I don't know what the words even mean in Italian, but I would say definitely try this out. This is actually helping my brain with thinking about the words in Italian rather than English, and when I get stuck on something in German, thinking about it in Italian becomes my "comfort zone" so to speak. Keep in mind, I'm not doing this to "learn" German, I'm doing it to help me with Italian. I'm not sure if there has been any studies on if it's good or bad for you to practice this way, but so far I'm enjoying it. I'd definitely recommend starting a tree in a language that you don't know very well (but know a little bit of) from the perspective of a language that you're currently focused on learning (and have already at least done a decent amount of the tree). When you take your native language out of the equation completely, it really makes you think of learning languages in a different way. My head hurts now...
There has been a lot of feedback in the forum from folks who have done reverse trees of the courses they've completed (I finished Spanish from English, then did English from Spanish), and from those who have laddered. (For instances, I'm interested in taking a Japanese from Spanish course, if ever one arrives to Duolingo.)
I can't personally speak for the benefit of laddering because I haven't done it yet. But, I found the reverse tree beneficial. It reversed the translation ratio from 2/3 into English, to 2/3 into Spanish. It was a step up in difficulty after the first or second checkpoint. It was a good chance to continue the learning journey through Duolingo. :)
I think what you're describing is frequently called "laddering" in these parts: using a languages you've made some progress in as a basis to learn an additional language. I have done it and enjoyed it immensely and wish there were more opportunities for it for me as an English speaker. I've been looking forward to the Swedish for Russian course for this reason, but it's one of the many courses not moving all that quickly in the Incubator.
Usually the goal is to work on both languages, though, not just the base language of the tree ;)