I know your question is several years old now, but I would suggest keeping the mileage for each business separate, even if it is just for your own records. In my case, my vehicles are my business as I own a limo company, do Uber occasionally, and rent cars out on Turo. Mine is simple enough that both 3rd party incomes just pay my LLC directly, and it is pass through income for me anyway, but I do like being able to go back later, down the road, and see how much of my vehicle expenses were from each revenue stream, so I know how profitable they were and if I'm using my assets productively.
Many years ago I got a paper mileage log from an author named Sandy Botkin. He has tax books available on Amazon for like $20, and I highly recommend buying the one that fits your particular business, as they have saved me thousands over the years. Anyway, in his paper log, he divides each trip into either business, commuting, or personal use. When I originally made a spreadsheet, I literally just copied his paper logs into Excel, since that was the format I was used to. Now that I'm self employed, I eliminated the commuting columns since I have nowhere to commute to, but I added columns for each revenue stream. Then when I do my taxes, I just add the columns together to give me my total business mileage.
Sorry, that was fairly long winded, and only halfway answered your question, but I figured it might help with bookkeeping going forward, even though it didn't answer the original question on how to file. But if you did decide to file mileage separately, it would be easy to figure out what percentage of business use was attributed to each business.
