Canadian needs to fill out international student forms for admitted program, but is still waiting on decision from top choice

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I've been accepted into my 2nd choice in an American institution, and asked to submit my I-20 financial guarantee, etc. (which will clear).

I'm still waiting to hear from my top choice--I sent an email asking when a decision would be made, but received no feedback. Should I begin the government papers for my second choice so I can register for courses, or do I need to wait it out and commit to a program first?

Any reflections about the CDN to USA process would be appreciated.