there are millions of ways.
1rst Using Geoset Anims, you add them to be linear/bezier or hermite, then you set values from 0 to 1, being 1 100% Visible, and 0 0% visible, thus 0.5 is 50% visible. Don't use this technik if your geosets already use geoset anims.
2nd Using materials, you set the layer types of the material used by the geosets to blend, and then you set alpha using values from 0 to 1.0
3rd change the layer types of the material used by the geosets to any of these: Additive(good for ghost/diamond/ shiny looking objects), Add Alpha(like blend with 0.5 alpha) or to modulate(a dark version of additive). Just by changing the the layer type, it gives the effect, no need of alpha.