As the novel states, Vito split them off to reduce their opportunity to plot against him. Since Tessio was smarter (and a Harvard graduate), he caught Vito's drift right away. Clemenza's only concern was with the availability of restaurants in the Bronx.
Also, the novel states two other reasons:
First, Clemenza was more impetuous than Tessio, and needed to be kept on a tighter leash. That's why Clem lived at the Mall, but Tess didn't.
Second, it was to Vito's advantage that his foes think that Tessio's distance meant that Tess was some kind of independent operator, the better to have the foes underestimate Vito.
