How to file taxes on cryptocurrency if multiple sources of funding

Hi! I've been doing some investing with cryptocurrency on the side this year with both my money, along with my fiancé's money. Unfortunately, I haven't been keeping track of what I do with each person's specific money, so now it's all just in a giant pool.

 

In order to calculate our taxes, I was thinking of setting up a general partnership where we can both specify I put in '$xx' and she put in '$yy' and hopefully settle taxes through Form 1065. I bought some coins last year with my money while she started contributing this year, but haven't created the partnership yet.

 

Would this be an issue since the EIN isn't created yet, or can I retroactively say that our business was created earlier this year? Or if anyone has any other suggestions! Thanks!