If youre doing studio lighting, you can most definitely go with two SB-600's. Zack Arias is a promoter of "One Light" photography, and you'd be surprised what you can squeeze out of a single light (or two SB-600's shooting through an umbrella if you need more power).
Get another SB-600 and the appropriate lighting equipment and youre good to go.
While I agree with the above post you can very quickly hit a wall using hot shoe flashes in a studio setup.
If you are just doing indoor work in a defined space, a few speedlights might be all you need. but if you are called upon to work in larger spaces with multiple people or more complex lighting, they just will not cut it.
For the work I do there is just no way I could do it with speedlights as I oftern have up to 5 strobes in my lighting setup and running well over 3200 W/S of light (that would be 30 or more speedlights)