Yes, you definitely need to add more chromis or you'll be down to one before you know it.
Chromis are best in schools because then a pecking order can be established, and the sheer amount of fish in the school spreads out the 'picking' by the dominant chromis so that it doesn't overwhelm any single one of them.
However, when you only have two chromis, the dominant one bullies the submissive one constantly until usually it kills it.
They're fun fish to watch when they're small, but keep in mind that they're essentially a damselfish, so in time they'll get an attitude like most damsels. Their one redeeming quality is the fact that even though they get an attitude, it's only directed at one another, and not at any of the other fish species.