How broad the accepted membership is entirely depends on the union. There is no objective reason to always segment by profession. In my opinion having worked in the games industry for over fifteen years software engineers are treated very similarly to other disciplines although paid better and it would make a lot more sense to work together with other disciplines than trying to each form a union separately.

As you say unions often have reciprocal agreements or are even part of larger umbrella organizations for example the TUC in the UK.