You can have more than one は in a sentence. There are situations (for example, when は is used for contrast) where it's quite common.
In this case, I believe it parses as:
もう片方の情熱は「お前の意志は受け継いだ」とばかりに膨らんでいった
(~とばかりに is a grammatical form which takes an embedded quote, like "as if...", "as if to say...")