If you're trying to use a normal "car" tire gauge, you will have a terrible time (without the fancy extenders, at least).
You need a tire gauge with a dual head. One faces frontwards for the inner tire, and one faces backwards at an angle for the outer tire. Similarly, there are dual head inflation chucks for compressors, which are also nearly necessary.
I found that a dual head gauge with the frontwards facing head in line with the gauge body was much less inconvenient for the inner tire than the more common type with both chucks angled. You can find a variety of models at a truck stop (or the usual online sources).
It's rather important to keep the tire pressure on each pair pair of dual tires equal (or as close to equal as possible); I try for no more than 1 psi difference here. It's not quite so important for the two pairs to match each other, though of course that's the general goal and they shouldn't be greatly different.
