There's a limit to the bonuses that a single piece of armor is able to give, but one piece of armor having a maxed bar in a defensive stat won't stop a second piece of armor from giving additional benefit.

If you look at your character page there's an icon in the lower left that'll show you the combined defensive bonuses you have. It counts the total stats from both armor slots (and trinkets, if you have them equipped).

Also, a Unique Variant that says "Maximum" doesn't actually max out the stat. It just gives the maximum possible bonus that a Unique Variant is able to give on a single piece of gear, which in some cases might cause the bar to max out when it normally wouldn't without the UV.

There's also two separate icons that count the combined effects on your individual weapons on the bottom next to the one that shows the combined defensive stats. If any of the effects there show that your weapon has "Maximum" in anything, then that's as much as it can get.