So I've got 20xp sitting in the bank and nothing that immediately excites me. Is it worth spending it on a second spec? If so, what all will I get out of it?

Do I get increases to the chosen skills?

So I am a Sentinel, and take something else with Stealth, do I get the new rank in Stealth, or does everything stay the same since I'm already spec'ed in it?

When you buy a second Specialisation that Spec's "bonus career skills" become career skills for you (saving you 5pts per rank on future skill-rank purchases). You also get the option of buying talents from that Spec's talent tree.

As Stealth is already a career skill for you, buying a Spec with Stealth won't help you be more sneaky. If that's all you're after, just keep buying skill ranks (5pts x new rank for career skills [max.5]).

When you take a second specialization, that specialization's skills become career skills for you (note that the base Career's skills do NOT become career skills). So, for example, if you picked up Aggressor, the Aggressor's skills would become career skills, but the Warrior's would not.

However, you do not get to pick "free" skills when you take the spec, as Col. Orange said. That's a one-time deal for your first spec.

As Stealth is already a career skill for you, buying a Spec with Stealth won't help you be more sneaky.

It might depending on the talents offered within the tree, but with only 20XP you won't be able to buy out of career and I don't think Shien or Artisan really add too much for a Shadow focusing on Stealth.

Personally, I'd buy a Force power; either Misdirect or Sense. Both have the potential to be useful and fit the flavor of a stealthy character.

As others have said, the point in buying a new spec is to gain a discount on the specialization's "bonus career" skills, as well as gain access to the talent tree. This can be very useful, but choose wisely, as the cost increases quickly.

Got a character sheet handy? Or a statblock for your character current?