Let T be the Tom' speed and S be the Sam' speed.

    (the same consistent units of speed are used).


Then from he condition,  

S = 2T         (1)   ("It takes Tom twice as long as Sam to drive from A to B.")
S + 2T = D     (2)    ("If they start driving towards each other at the same time, they will meet one hour later.")

where D is the distance between A and B.


If instead they started driving in the same direction (with Sam driving towards B), 

then T has head start D, the relative speed is S-T  and the time before Sam will catch up Tom is


time =  =          (replace D with S + 2T, according to (2) )

     =  =          (replace S with 2T, according to (1) )

     =  =  = 4 hours.


Answer.  It will take 4 hours before Sam will catch up Tom.