There have been some really good posts here that made me stop to think about the "cup".
My sensei never had a problem with anybody cross training -- as long as when they were in the aikido class, they did only aikido. He also said that when we were in the other martial art to do that art , and not aikido. Otherwise you were short changing yourself.
He always said that to make progress, you needed to live in the present moment - and study whatever you are doing at the moment.
I don't think that the cup gets bigger. Maybe we have many different cups that are easily interchangeable. Maybe we need to learn to be able to tell the difference between cups and fill each one accordingly.
Unfortunately I have seen many people walk into a class thinking that they know something about anart just because they have studied another. I think it would be very good if the student walked in with a solid foundation in the basics of one art, and openly accepted new concepts without trying to revert back to the other art. Then, and only then, do I think that cross traing is good.
Otherwise, the teacher will always be telling the student to "empty the cup".