Martial arts instructors and students are divided into two camps, those who think training in more than one art is fine, and those who hate the idea.

In a lot of traditional martial arts like Karate for example, there has been somewhat of a change which has been brought about by MMA (Mixed Martial Arts) and the success of people trained in multiple arts in events like the UFC (Ultimate Fighting Championship). Now there have been a couple of UFC fighters with some Tai Chi training but most Tai Chi instructors ignore that, as they ONLY teach Tai Chi for health and some of them don't even know or understand how it can be used for fighting.

I'm no better, for a good number of years I learned Tai Chi without any understanding that it could be used for self defence or fighting (There is a difference but that's another topic) and I was 'enlightened' (painfully so) when an instructor showed me just how devastating an art Tai Chi can be.

The Tai Chi instructors who dislike cross training are against their students training in other types of Tai Chi - which is OK if the student is a complete beginner, but not otherwise. They are against them doing 'hard' martial arts like Karate or Judo, because they say, they use Li (strength) not Chi (internal energy). They are definitely against weight training. And finally, and maybe a bit cynically, it seems to me, that many of them are against anything that might either be more interesting to a student or which might result in them losing income.

I have always been an exponent and a proponent of training in anything you want to train in, and have always said to my students that it is completely OK to go and see other teachers of Tai Chi (or anything else). Basically, freedom of choice is important and no self-styled master of martial arts should tell you otherwise.