Put the feature lines in different sites and they won't affect one another.
I think the goal is to maintain the slope from the EOP feature line so that it will hold that slope but float vertically when the EOP elevations change. If the feature lines are in different sites they won't maintain a common elevation at the vertices they share and thus there would be a bust in elevations at those locations.
Really there is no way to lock the slope of featurelines in C3D. There may be workarounds such as using gradings, corridors. etc.
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