Is it possible that SSID overlay each other ?

When I look with well known SSID scanners (e.g. NetStumbler) for available wireless LANs
then I cannot see certain nets. However according to statement from other users they should
be available.

In order to be able to connect to an underlying SSID (net) in the same channel I must somehow disable
the dominating visible SSID. How can I do this ?

Does someone face such a situation in the past?

Can I otherwise force my WLAN to connect to a net which is not visible but existing?

Gunter