I love it when I get a chance to tell this story! I had a boy & girl training at the same time 3 years ago and the girl peed in the small sit down potty and the boy trained standing up at my toilet. One day I asked the little boy to give the little girl room because she had to sit down to pee. He said 'I know, because boys have weiners and girls have petunias so they have to sit.'

I went through it again the next year with a girl and a boy who used the same system but it was because I could get them both to go at the same time. Now I don't have to use the small potty any more because the girl now uses the regular toilet.

Anyway, to answer your question, no it doesn't matter to me if mixed genders are training. At daycare centres they use the same bathroom too I believe.