Ben Dinh and Ben Duoc are about 15 km apart and most visitors choose one rather than both. The sites are self-contained: once you arrive, everything is walkable within the grounds. Tours typically cover Ben Dinh; reaching Ben Duoc independently requires planning ahead.
Within either site, a guide is strongly recommended. Both sites have English-speaking guides available at the gate for hire if you are not on an organised tour. The physical experience of the tunnels is memorable regardless, but the history of what happened here — who built these tunnels, under what conditions, against what — is what makes the visit coherent. A good guide provides that context. It is included in most tour packages; hire one independently if you have arrived on your own.
The outdoor exhibits at both sites involve walking on uneven ground in heat and sometimes mud. Comfortable shoes and a hat matter more than anything else you bring.