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The Natural Philosopher

wrote:
If the main vent is near enough, you could get by with an air admittance
valve on the short stack - or a shortened version of it.

Maybe. The forward air pressure from flushing the upstairs loo may be
sufficient to "blow through" a trap elsewhere - e.g. kitchen sink. That
may be why the second vent was put in.

Agreed.

A lot depends on what is downstream..if the neighbour has a stack...

The general principle is that air valves work upstream of at least one
free vent.

In my case I have tow fairly long branches one of which is stacked and
the other is valved. The upstairs toilet has not yet blown the traps on
the downstairs stuff on the same branch.