They had Flash on Supergirl last week(8-9 days ago). I was disappointed on how they did it. They had them from different Earths and Flash traveled to Supergirl's Earth.
Superman has made a few blurred appearances on the show. I think they are trying to tie Supergirl to the Superman movie world. It has already been said that whenever JLA comes out they will not use this Flash(tv) but create another one. So, I wonder if this is how they explain this.
Funny, in order for me to sleep I sometimes have to think up stories in my head. I've been doing Multi-universes for several years. Now TV DC has been doing this as well.