Security of dual ssid

I use two SSIDs in my wlan network. For one WPA2 encryption is used and the other is kept as open for guests.
In such a case is the whole network unsecure / security vulnerable.

Thanks...

It depends on your network. Can you share some more information about your network, setup, size, etc...

It would make sense to isolate your guest wifi and possible put it into a DMZ if you can.