In your situation, all you can do is use the information you have and come up with a reasonable computation. Just do the best you can.
You don't have to spend a lot of time doing it -- just create a document, list your income, and make what you think is a reasonable allocation of income per city. Note on the document why you're doing the allocation this way -- basically, the information you've provided here. In the event you don't remember the circumstances behind the computation, this will be helpful.
Then, use those amounts on your return(s) and keep the document with your tax records. It's unlikely that anyone from the state (or city) will ever need to see it, but if they do, you'll be able to show that you made a good-faith effort in your computations.
