I both AH to play 2p mostly, and was also heavily against the concept of having to get a 2nd core, but I was fortunate enough to have 2 other friends who were interested in trying it out, which forced my hand into getting a 2nd core now for 4p. They're not in the same city anymore, but now I have the 2nd core for our 2p games, which I now feel is 100% worth it if you are well invested in the game. The question is then whether you are invested enough in the game.
If you are unsure, get one of the deluxe expansions (Dunwich, Carcosa, or Forgotten Age), which add new player cards. Play through the new scenarios, get the additional 6 mythos packs (which add player cards too) to complete the 8-scenario campaign. 1 core set should be enough to play through it on easy with some moderate amount of customization. This will give you a good idea of whether or not you want more customization (if you do, get 2nd core). If you're ok with playing through the campaigns on easy for the story and less for the customization, just get new expansions instead.
I personally love the deck customization. I consider it as important as the narrative aspect of the game. Knowing what I know now, I would still have bought a 2nd core even if I didn't have that 4p opportunity.