For full context: http://www3.nhk.or.jp/news/easy/k10011273631000/k10011273631000.html
The sentence in question: 水族館は、オウサマペンギンが外で散歩をすることができるのは気温が低くなる冬だけなので、いい運動になると言っています。
My attempt at translation: Literal: "Concerning the aquarium, concerning that the penguins can take a walk outside, they say that it becomes a good physical exercise,because there is the alone temperature-lowering-winter." More elegant: "The aquarium says that it becomes a good physical exercise that the penguins can talk a walk outside because winter alone is bringing temperature down."
So, First, as the title suggests, I don't really know when two topics are allowed in a japanese sentence. I also don't really know how to handle them, how they relate to each other, wether one topic can be subordinated to the other etc.. I think I understood the sentence contentwise, but I've little idea how the grammar actually works.
Furthermore, probably because there are multiple topics used to build the sentence, there is kind of a lack of transitivity...^^ What confused me the most was 気温が低くなる冬だけなので. I think the ghist of this part is that winter brings down temperatures, which is a good thing for the physical well-being of the penguins. However, 冬 doesn't take direct influence on 気温, but instead it is an attribute of 冬 that expresses "becomes lowering temperature". The problem with hereby is that 低くなる itself is also a bit confusing since 低く is a noun and not an adjective. I can follow this logic in some cases, but in this case its a bit hard because it is not something along the lines of "The swine becomes a steak" (sorry for the bad humor^^) but more along the lines of "The sun becomes dark". This difficulty I have stems from the entry on http://jisho.org/search/%E4%BD%8E%E3%81%8F where 低く is categorized as a noun or suru verb, however the two meanings are no nouns Oo Therefore, my translation doesn't reflect a "NOUN becomes a NOUN" structure but a "NOUN becomes ADJECTIVE" structure. Because of that, I feel like I didn't really understand whats going on in this sentence Oo
I also don't know if I understood the use of だけ correctly. I think that here it is used to express that only through winter such a drop in temperatures can be achieved, at least for a "walk outside" (One could also put the penguins into a cooling chamber for example).