I have two HC-SR04 sensors working close together on the same frequency. Using the NewPing library, admittedly, but I think the principles apply.
Same frequency is not a problem so long as you allow enough time for the echo from one ping to arrive (if it is going to) before starting the next ping. In the NewPing documentation, there is guidance on the timing on this.
In your application, I had inferred (maybe wrongly) that there was an in lane to the car park and an out lane, and your sensors were positioned to bounce of cars going through each lane. In that case, crossover echos may not be a problem, and if you sequence the pings correctly, won't be a problem.
So, as a start, how about switching the order of your code around as per my earlier post, putting a few seconds delay for debugging purposes between the two blocks of code (sensor 1 ping / echo and sensor 2 ping / echo) and then testing to see if you are detecting cars and if the counter arithmetic is OK?
Cheers
Ray