Wooden blocks friction problem

1. The problem statement, all variables and given/known data
Two similar wooden blocks are tied one behind the other and pulled across a level surface. Friction is not negligible. The force required to pull them at constant speed is F. If one block is stacked upon the other then the new force required to pull them at constant speed will be approximately ?

2. Relevant equations
f=μN
F=f

3. The attempt at a solution
I can't understand if they are connected by a string or something else. If a string is present then I will have to apply a Tension force. If they are just simply connected by some other means then the contact force has to be taken into account. By either way I am not able to get the answer.
 PhysOrg.com science news on PhysOrg.com >> King Richard III found in 'untidy lozenge-shaped grave'>> Google Drive sports new view and scan enhancements>> Researcher admits mistakes in stem cell study
 Blog Entries: 27 Recognitions: Gold Member Homework Help Science Advisor hi utkarshakash! welcome to pf! suppose the two blocks have mass m (each), the coefficient of kinetic friction is µ, and the tension is T … since the acceleration is 0, what are the equations for the forces on the two blocks?
 Thanks. I just got my answer.