It’s definitely a crossover!
It won’t be a crossover with Doctor Who. We did attempt one, long before Brothers Consulted was conceived. It was called Brothers Spaced and I don’t think we got more than a chapter into it because Dean and Zepheera are a gigantic clash for such little people.
( @borrowedtimeandspace: seriously, they just shouted at each other for like three pages. Doc would be so fret if he discovered the kiddos though! )