When I began my "car maintenance fund" I put $1000 in a separate savings account for that purpose. Then I put in $100 every month. It has been super nice to automatically have enough money in the account for things like service, new tires, a repair... I love not having to scramble for the funds.
I am thinking of upping the amount to $200 a month eventually. My car is a 1997 Subaru wagon with 225,000 miles, and while it has been a great, reliable car I know it will not last forever. But saving $100 extra a month right now I will have enough to cover if it needs a major repair, or enough to put a healthy down payment on a newer car when the time comes years down the line. Kinda like an advance car payment.

If I were in your situation I would probably start with a repair fund of $1500, and then put in $150-$200 a month to cover both car's needs. Starting with money in the account isn't necessary, of course, but I was worried about having to do a big repair when I'd only been saving for a month or too, and I wanted that padding so I could take care of problems right away without using a credit card or my emergency savings.