Is Mind Probe good enough to run as a two-of? I guess my real question is: should I get a second Kylo Ren starter set, or is one probably enough?

I think it's definitely good enough in some decks. Holochron makes it essentially free. Poe can resolve it with his special. 

However, I think it's fair to say that the longer the game goes, the more options there will be and the more likely there will be other suitable alternatives.

I think a second Rey starter for a second Force Throw (and maybe a second Finn die) is much more valuable than a second Kylo starter ... but I personally bought a second Mind Probe as a single (in addition to the second Rey starter).

The more I play, the less I run 2 Probe (or 4-cost upgrades in general, for that matter) outside of Poe. Since you can go through a huge part of your deck in most games with a 30 card deck and drawing up to 5 cards each turn, you don't need to double up on your end game cards as much as in other games to have a good chance at seeing them. 

As with Poe, Holochron makes Mind Probe more than an 'end game card'. A first round Mind Probe is very doable and very powerful ... so you're motivated to run two in decks with Holochron.

So, I'd say your statement is true of hero decks, but perhaps less so for blue villains.