Well I know this will happen, is normal and life will be boring without problems, also problems makes success more satisfying when it's achieved. 

As I advance on the circuit I also do tests, the first test was with only one VCM connected to LM298HM, at that time I don't see any aparent problem, but when I did test with two VCM connected to L298HN I noticed a voltage drop on the ouputs of L298HN to both VCM from 4.5V to 3.23V, I also measured voltage drop at output of the variable power supply (LM350K based), this tell me that voltage drop was on variable power source, so I made a test connecting both VCM in parallel direct to P.S. output, test conditions was:

• Vin = 19.1V
• Vout = 5.11V (without load)
• Vout' = 3.51V (with two VCM connected)
• RL = 5 Ohms (both VCM are 10 Ohms connected on parallel)
• IL = 0.690 A  (measured)

If you see there is a voltage drop of 1.6V on Vout when load is connected to P.S. I need to check the datasheet to find what is wrong here (or if this is normal). Also there is a voltage drop too if I use only one VCM but is low enough to overlooked it on first tests, so if I connect three VCM then the voltage drop will be more than 1.6V (however I don't tested it).

This is the used circuit (from TI datasheet):

And this is the implemented circuit:

I don't think this problem is thermal related because LM350K use a large heatsink and the changes on Vout are fast as the change of load. Well I will start here the analysis to find the possible problem, wish me success.