I have duals on my MH and always put the same number of blocks under the inside wheel as the outside wheel. However, if I were in a situation where I could only put blocks under one wheel, I'd put them under the inside wheel rather than the outside wheel. By putting them under the inside wheel, all the weight is supported by the wheel lugs as close to the hub as possible and the outside wheel is allowed to hand free. Putting them under the outside wheel moves the supported weight a bit further out on the lugs while the inside wheel hangs free.