It depends. If you are looking to file as a qualified joint venture (I have an additional answer with more information on this), you can choose the percentage you wish. You are correct, however, in that you do need to prepare two Schedules C and divide everything by the same percentage if you wish to claim the joint venture as a business for each of you. If you have never done a joint venture, you can choose to file just one Schedule C, but only one will get credit for the work (and pay in Social Security and Medicare taxes). Your other option is to do a partnership return (which for most is even more complicated).
Here is some additional information on this subject: https://ttlc.intuit.com/replies/5717482
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