Before finding my sifu I trained in Shotokan under the master I mentioned before. However, when I found my sifu I started WC training under him and continued with Shotokan. Sifu had no problems with it. Eventually, I realized that karate training was hindering my progress in WC and I discontinued. Sifu said that he had no problem with me training karate at the same time because he knew that I would eventually realize that it was hindering my kung fu progress. The concepts of WC I train are just too different and so are the energies (internal) and the softness and the flow
WC and shotokan do NOT compliment each other, quite correct.
Karate and Kung Kuen are ok, but to be honest, training in two different striking systems at the same time doesn't make a lot of sense.
If they comliment each other, what's the point? and if they are opposites it is better to do one when you have a firm grasp of the other.
Of course cross "testing" is a different thing.
Psalms 144:1
Praise be my Lord my Rock,
He trains my hands for war, my fingers for battle !