Well BS10927 its never a "BAD" to cross train in my opinion, regardless of what styles you do. There is something to be learned from all systems. In regards to Wing Chun and Muay Thai, I would say that the ability to train both the styles is entirely up to you. I train Muay Thai, and have also trained Jeet Kune Do. JKD borrows alot of the trapping techniques from Wing Chun. I have personally found that some techs can be effective within the Thai Clinch, but it really depends on your ability to flow from one technique to the next and your personal preference. My advice to you would be to train both and see how they work for you.
i agree, crosstraining is never bad in & of itself. however, if i were to divide my attention between two arts i think i would train one striking style & one grappling style. it will give you a broader range of techniques & you will be less likely to confuse the two.