> Are two strobes side by side at half power equal to one at full power in manual mode?
Of course. Two identical lights will supply twice the power of one, and twice the illumination is a one stop difference in exposure. Similarly, 4 lights will be 2 stops brighter than one, and 8 lights 3 stops brighter.
This is assuming, as you stated, that the flashes are set to manual, and that they illuminate the same area (i.e. are the same distance from the subject and aimed at the same point). That would not be the case if the strobes are aimed differently, for example, to provide wider lighting for a group shot, to evenly illuminate a whole room, or split between the subject and background, etc.