It is best to put blocks under both. Raising one tire without blocking the second could cause the weight for that corner to be distributed to one tire only if the inner or outer dual is raised off the ground. Raising one tire off the ground without raising the other is basically cutting you surface area for that corner's weight distribution in half.
US Army (Ret)
2020 Entegra Accolade 37TS
2019 Jeep Grand Cherokee Trailhawk (Toad)
FMCA - F432054