well im not really sure if u could 2 man instances.. but im not sure that is really what u meant.
The only problem 2 boxing does not really increase productivity as much as 3+ does. As you may know multiboxing kinda does inhibit some ability to preform as you could with just playing 1 character, that is why more is better because of the increased dps. I think if u can go for 3. Make one a tank and the other dps. pick up a healer and a dps each time you run a instance to lvl.