And can two servos with 20kg cm stall torque move a wheelie with 15kg?
Stall torque is not the only important parameter, but assuming you didn't put 15kg on their axles and they each has got 20kg-cm, yes, they would be able to move it, very slowly, on a level plane, as long as your wheels are less than 5.3cm in diameter - make the wheels 5.4cm and it won't budge.
The output shafts on a regular servo won't like the load or the direction of same, but it could be coupled to an axle mounted in bearings.
Servo motors are ridiculously small though and get their strength at the cost of slow speed due to heavy gearing. If you want your vehicle to move in a fashion that'll keep you from dosing off, use gear motors.