A mechanism is disclosed for providing horizontally split vias in printed wiring boards (PWBs) and other substrates. In one embodiment, the substrate includes a plurality of insulator layers and internal conductive traces. First and second through-holes extend completely through the substrate and respectively pass through first/second ones and third/fourth ones of the internal conductive traces, which are at different depths within the substrate. Photolithographic techniques are used to generate plated-through-hole (PTH) plugs of controlled, variable depth in the through-holes before first/second conductive vias are plated onto the first through-hole and before third/fourth conductive vias are plated onto the second through-hole. The depth of these PTH plugs is controlled (e.g., using a photomask and/or variable laser power) to prevent the conductive vias from extending substantially beyond their respective internal conductive traces, thereby horizontally spitting the two conductive vias plated onto each of the through-holes. This advantageously increases wiring density up to 2×.