I crosstrain. It works well with sparring and such, and certainly the extra training helps with your stamina/fitness. Problem is, my TKD instructor now tells me that my basics look more like Shotokan (cos of the deeper/stronger stances) and that my forms look too much like Kung Fu (again cos my stances are now deeper - but I don't do Kung Fu anyway!). Also, having learnt of some new principles in Shotokan, I'm starting to doubt my TKD and wonder if I should apply these ideas or not (for instance, in Shotokan forms one turns their head to always look in the direction of the technique, but in TKD forms we've always trained where you keep the head still - which makes less sense to me). I'm not sure whether this change in style is a good thing, but I think it's probably good not to try and confuse styles in class.