It’s the same approach… two plugins go into a 4ch mixer, then you need two widgets which are connected to the according “volume” parameter of the mixer channels. Then learn both to the MOD-wheel, but for one of the widgets invert the MIDI input (see the manual Widget Properties inspector (gigperformer.com) item “MIDI Tab → Invert button”).
That way the widgets will move in opposite directions… voilà a simple crossfade!
In your place i would use another two Gain-blocks which would go between the plugin and the mixer, then use an additional two knob widgets which should be connected to the “Gain” parameter of the gain blocks. This will allow you to adjust the volume of the two plugins separately, while the mixer block will just control the mix-ratio.
You may also use the forums search for “crossfade” i am sure you will find some additional stuff. 
Here is an extensive thread about how to crossfade, with diffrent solutions:
Stereo cross fader - Plugins - Gig Performer Community
And here is another Scriptlet-based option (if you want to use that):
[Gig] X-fader - crossfade two volume parameters - Gig and Rackspace Files - Gig Performer Community