I use a Milton gauge (Amazon). These are not cheap but they are heavy duty. Only rotate the fronts - I don't mess with the rears. The inner wheels on the rears are steel anyway (Ford). I did add extensions for all SIX wheels to make things easier. The inner rears got the bendable brass/copper (?) kind. Make sure you have metal valve stems if you add the extensions.
Don't trust the gauge on any inflator - buy a GOOD gauge.
I do not understand how the inner wheels could possibly carry more weight if both duals are aired up to the same psi.