Descriptive geometry used to be taught to engineers, not so often now that we have computer drawing software. The idea is to project 3D objects onto TWO half-planes, then flatten the half planes into a sheet of paper. There is a redundant dimension in the representation.

This can be exploited to visualize 4 dimensions: simply project a 4D object onto 2 planes. Descriptive geometry with both half-planes independent. One can go up to 6 dimensions by projecting onto 3 planes, or onto 2 volumes. The trick does help somewhat.