Can I install one 20-013 per window?

A quick learner from LA California says:
Good Morning, We are getting ready to install the 20-013 on our double hung windows. Should we install ONE lock per window - or two - one on each side?
Thank you.
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Paul from SWISCO responded:
It is recommended that you install two of these vent locks per sash. That would be one 20-013 Wood Window Vent Lock Set per sash.
A quick learner from LA California says:
Thanks, Paul! Appreciate the help.
A home owner says:
OK....this means that the top sash cannot be pushed down either when the ball is set to lock...right? I have double hung wood windows.
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Tom from SWISCO responded:

Yes, that is correct. The 20-013 is positioned on the top sash, so it would bump against the bottom the same way the bottom would bump against the top. See the diagram below to see how the 20-013 is positioned.

Demonstration of how 20-013 is operated
Demonstration of how 20-013 is operated
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