Cross training is 100% legal in every state.
Cross training is, as a general practice, a good management technique. If you think about it you will see why - if you can't, I'll explain it to you.
As you have described it, you are going to have a hard time making a claim of discrimination stick until or unless someone is fired or disciplined for poor performance on one of these cross training duties AND it is one of these "old and sickly" individuals you mention.
I'd like to ask one more question. One of these positions the regular workers are paid more than the department we are in. If we get certified in this department is the company required to compensate the difference even if we work there once in awhile?