cutting at zero crossings in comp mode?

If you're splitting with the comp tool, there will be automatic crossfades created at every split with a default duration of 5ms (IIRC) set in Preferences >Customization > Editing. You can adjust the width of individual crossfades after the fact if necessary by hovering over the split to get the crossfade tool, and then clicking and dragging up and down.
 
Since it's unlikely that waveforms in different takes are going to have coincident zero-crossings, it's not possible to ensure all splits are at zero-crossings (or that the signal is moving in complementary directions on either side of the split for that matter). Splitting at zero-crossings is no magic bullet anyway; it's more important that you split at points where the surrounding amplitude is as low as possible, and let the cross-fade take care of the rest.