Well they hold 6 plants each, so I'm thinking two 400 watters would be better for two Emily's Garden-type set ups. Or like me a 400MH for veg and a 600HPS for budding. Move some out of the flower room to dry, and at the same time move more into the flower room that are vegged and ready to flower. That's what I like. However if you do a real short veg like you talk about, the plants would normally stay pretty small, so one 400 might be OK. Of course more light is nearly always better.