[HUE]

If the one student has a lot of grammar problems, it wouldn't hurt to go through grammar and drills with the both of them. Obviously, though, when it comes time to use a freer activity and put the language in practice, the stronger student will have a lot more to say. But that's fine, because both will be able to get the benefit of some new material and/or review material, and then apply it according to each individual's ability level.

Or consider having the stronger student teach/help the weaker student. Using this technique from time to time challenges the stronger student, and can act as a confidence builder for the weaker student -- sort of an "he did it, so I can too!" In addition, the stronger student will be able to explain problems, having faced similar ones, too.

Hope this helps!