Welcome to the forum!
Its great to hear someone new to the business learning about how to run theirs. The forum is a great place to learn and share information.
As for taxes, I hesitate to provide information without stating the final authority is the IRS. You might wish to go to that website for confirmation. With that said I record both for different reasons. The IRS requires a log with beginning and ending mileage for the year (I know you started in the middle, do the best you can) and starting and ending mileage for each day it is used for business. I also keep the total number of business miles for the day. I do that so that I can take advantage of the spreadsheet summation function at the end of the year to total my business miles for the year.
There are two ways to claim mileage. The first is miles times the number of cents allowed. In 2014 it is 56 cents per mile. So at the end of the year if I claim 3023 miles, I multiply that by 56 cents to get my mileage deduction. The second method is to total all expenses for the car for the year (gas, insurance, repairs, maintenance, taxes, etc..) and divide that by the percentage of miles the car was driven for business. You probably can see why most businesses choose the easier first method.
If I'm not mistaken, the IRS requires a log for each vehicle used. For you, because you use two cars I think you might want to have two columns, one for each car. That will make more sense as you enter it and help avoid errors.
Have fun with it.
Happily shopping Rhode Island and nearby Massachusetts and Connecticut