It's about more than just the pure logistics of getting there and back. Double the dates for all school things.
Like what ? There just aren't that many things that parents need to attend at secondary school
Two lots of collections
What do you mean ? Monetary collections, or do you mean picking them up from school ? I mean, I can't see why there would be any monetary collections, but if there were, then two pupils will mean two amounts, or, if you mean picking them up occasionally after a late practice or something, then the odds are they wouldn't both be at the same practice anyway.
and PTAs
Quite unusual for there to be particularly active PTAs at secondary - but even if there were, in the very unlikely event at both schools, then there is no compulsion to be part of it
and parent evenings
I would consider it MUCH easier to try to see one dc's teachers one night, and the other on a separate night (likely if at different schools) than trying to meet the impossible task of trying to get round both sets of dcs' teachers during one (usually very time limited) parents' evening. That would count as a reason for it being better to be a different schools, to me.
and all the rest. Different shops for uniform?
Round here, there is one shop that sells uniform for all the secondary schools. Others that sell generic stuff like grey trousers and white shirts, but you aren't exactly going out and buying blazers that often, even if you lived in a place where different schools meant you had to go to different shops.
Two sets of school rules and systems and all the rest
but that is for the pupils to be aware of / manage, not the pupils.