Filter stacking question...

All other things being equal (seeing, tracking, CCD response etc),
would there be any difference between an image derived from a 10
minute exposure through a narrowband filter (e.g. HII or OIII) and one
derived from a 20 minute exposure through two stacked filters of the
same type?

I ask because I wondered if stacking narrowband filters could improve
contrast between blocked and bandpassed wavelengths with the only
downside being longer exposure times needed. As I only have one of
each flavour - I can't test it.

Cheers
Beats