The problem with the Knicks is that they don't really have the assets for a second star, because every single team that is selling a star will demand the best picks/package the Knicks can offer. And if the Knicks don't offer it, then a team like the Cavs will swoop in and offer it because they don't need to save their best stuff for one more star - they already have the core of their roster set. In essence the Cavs have the assets for 1 star and a few good role players(think Convington, Jerami Grant types).
You know who has the assets for 2... maybe even 3 stars right now? Ironically, it's the Jazz. And because tons of those are far far into the future so they will be available for a while.