So Dualverb is a pretty nifty algorithm as it lets you play around with two separate reverbs, A and B. It will take your input and apply each of the reverbs to it in parallel. The Decay knob will let you set the decay of reverb A and the Xnob will let you set the decay for reverb B. If you would like to freeze one of them, simply set one of the knobs to freeze and then the other to some other decay time.
What's your setup like? Are you using stereo or mono out? Check the FxMix knob; if you're using stereo out, then this will change which channel each of the reverbs will come out of and in mono this will determine how much of each reverb is sent to the output.
What do you mean by save, as in when you try to save the preset it disappears? Or are you trying to assign one of the freezes to the HotSwitch?
Hope that helps,