I need to learn German for professional reasons to a relatively high level (say C1 on the standard European scale). At the same time (or within a couple of years, say), I would like to learn Yiddish for personal reasons. Presently, I'm a beginner in both languages, but I can read them decently. (That is, 20 minutes for a page of German and maybe 40 for a page of Yiddish, where the Hebrew-origin words really slow me down.)
I already have experience learning languages. In the past, I learned Spanish after learning French and Italian. The result was that nothing happened to my French, which I spoke very well, but I became unable to speak Italian, which I had been fluent in (perhaps B2), though much less than in French. The little Yiddish I've learned has likewise erased, for now, the limited ability I previously had to speak German (as opposed to understanding it).
My questions are as follows:
1) How difficult is this going to be?
2) What can I do to make it easier? (For example, learn Yiddish with textbooks in German that draw attention to the differences?)
3) Would there be a big advantage in delaying learning any more Yiddish until my German is good? (In that case, I'll certainly lose what I've acquired so far.)
Note that there is an additional difficulty here besides the similarity of the languages. That's that I expect I'll have relatively few interactions with native speakers of Yiddish, and many with native speakers of German. I already talk to myself in Yiddish about a hundred times more often than to anyone else. My cat comes in second place after me, and at least he doesn't already speak English, so, if anything, Yiddish is more useful with him.