When two gradings cut each other, for example mine corridor grading and a lateral ramp grading, you can use the Define target aliasing tool to use one of the two gradings as a target for the corridor containing the other grading to obtain the 3d linear between those. Later you can use the Corridor tool to eliminate what you dont need.

I think that a good improvement would be to obtain the cutting 3d element as a 3D Linear element Ruled, then you can use this element as a target to define the extension of both gradings.