I think he didn't specifically mean the hearts sleeping inside Sora (Ven, Roxas and Xion). Remember what Ansem the Wise said when Riku dived into Sora's heart in order to wake him in DDD: ''A boy like him, who has touched so many hearts''. Sora has the power to connect very easily with people and make friends, that is why Yen Sid says he is the key. Even Riku said something similar when he was a kid at the end of BBS, when he told little Sora to open his heart and listen and that he was the only one who could, which Sora did and met Ven's heart again (I don't remember the exact wording but it was something like that). Young Xehanort is aware of this and by saying ''hearts joined to yours'' I think he meant, in a broader sense, ''find those connected to your heart and gather the seven guardians of light''.
Probably referring to either Ventus, Roxas or Aqua, because the organisation had interest in using Xion and Terra as darknesses. That makes sense because gathering the lights and darknesses was Xehanort's goal in order to forge the X-blade.
But who knows...the word 'joined' is the problem here. It can be interpreted in one way (metaphorical connection = friends) or the other (literally joined, as hearts inside a heart).
But maybe he literally meant the hearts inside Sora's and was actually encouraging Sora to use the power of waking to wake them, because
Young Xehanort knew what would happen to Sora after ''wielding such power foolishly''.
Both interpretations make sense and are not mutually exclusive, he could actually be using such a vague wording so that Sora wouldn't notice what he was implying either way. Giving a poisonous advice while mocking Sora without Sora knowing what is going on really fits Young Xehanort. Is it a villain-esque line? Yeah, for sure. But a line which carries a lot of deep, hidden meaning.