Hi,
To calculate the forwards torque I would go about it like this:

When pushing another robot the combined mass is : 27+13= 40kg
So when pushing the kinetetic energy of the system is: 0.5*40*1.5^2= 45J (Assuming 1.5m/s max speed when pushing as walkers are slow)
So assuming you want to get them up to max speed in half a second, power required is: 45/0.5 = 90W
Forward force required is: 90/ average speed when pushing = 90/ 0.75 = 120N forwards force
Sounds like you have 3 legs so 120/3 = 40N per leg so torque per leg = 40*0.15 = 6Nm

Having done all that I realise it is all completely worthless as it doesn't take into account any friction or the driving force of the other robot, I think I've calculated the minimum torque to push the two robots when on an ice rink so not really very helpful.

I will have another go at it later when I can right everything out properly and draw a diagram as my maths teacher always advises me to do, it's quite hard doing it in an iphone.