Its because running 2 ACE DS1015s would draw a lot of current and is unnecessary. The 1313 is considered a top of the line throttle servo, as well as a good steering servo. The 1015 pulls 210oz-in in reality (not the 200 as advertised), and goes lock to lock in 0,1 secs, which is huge overkill for a 1/8th brake servo. We use them as brake servos on large scale cars.