Can you wear 2 sets of armor to get the multiple upgrade slots?
Ex. wearing a second skin under a defrex hide to get a total of 2 upgrade slots
Armor worn under armor has no rules for its functioning, so Second Skin wouldn't work for extra slots, barring houseruling. However, since the power armor question was closed as a duplicate, I'm answering it here instead.
Unlike Second Skin, there are specific rules for how power armor and light armor interact.
If you’re wearing light armor while in powered armor, you gain the higher of the EAC bonuses and the higher of the KAC bonuses between the two suits of armor, and you take the worse maximum Dexterity bonus and armor check penalty.
Since both have an effect, you should be able to utilize any non-stacking upgrade slots in both, with some restrictions. Since you use the power armor's move, and not your own, things like jump jets would not work in the light armor.
Specifically regarding force fields, things get a bit fuzzy. Pathfinder had rules about Temporary HP not stacking, and only the higher value applying, but I have been unable to find similar rules in Starfinder. So unless there's something specific I'm missing, shield upgrades fall into the category of "untyped bonuses" for stacking purpose, and would both function fine, with the outer shield losing temp HP before applying to the inner one.