Two bits per ROM cell
That technique is only viable for die cast (non-programmable) ROM. PROM is more flexible.

A lot more components have to work correctly for it to work, reducing yields. Each transistor in the ROM must fall within its margin, the resistor must be correctly sized, the comparators have to work correctly. 

Flash memory is being developed using multiple levels with more bits per cell according to wikipedia (https://en.wikipedia.org/wiki/Multi-level_cell)