As Tagteam mentioned, you would first need to confirm that the rental activity rises to the level of a "business" and that the office is used "regularly and exclusively". Personally, I would think it would be unusual for a person with two rentals to meet those requirements.
But if you meet those requirements, in answer to your questions:
1) I think either method would be allowed.
2) Split between properties.
3) If your home is your "principal place of business", then the miles should be deductible.
