How can I control two motors using only two wires?
What do you mean when you say 


  However, if I were to complete my circuit using the yellow wires in the above diagram, the other relay device would be activated and both motors would turn. 


Unless I am misunderstanding something here, it should work, provided you add the required circuitry (limiting resistors for the optocouplers). 

If you are in the "forward" position, we have the following circuit:



The left optocoupler is forward biased and it's LED will conduct current. The second optocoupler is reverse biased, and it's LED will not conduct any current. Hence the left one is turned on and the right one is turned off. If we reverse the supply, we turn on the second optocoupler, and turn off the first one.

We can actually get rid of one resistor by doing the following:



Now, I would like to point out that if you are using actual optocouplers, it's a bad idea to use them to directly drive the motors. Instead, use them to drive a MOSFET or other switching device, that will in turn actually switch on the motor.