If you mean just plain alfalfa hay pellets(not rabbit food pellets), then those should work fine. I can't see why you couldn't give them to your other bun as well if you felt an extra bit of protein in the diet was needed. If you felt either bun was on the lean side and needed an extra bit of protein, then you could add in a bit of the alfalfa hay pellets(or not if they are at a good weight on their current diet), but I would still feed timothy(or other grass hay) as well. I'm of the mind that feeding alfalfa hay exclusively to young rabbits isn't the greatest to do or necessary. Yes it can be helpful for their growth, but if you are already feeding an alfalfa based food pellet in unlimited(or close to) amounts, then they will be getting extra protein from those. You don't want too much protein in the diet as it can sometimes lead to obesity, excess ammonia in their urine, cecotropes left uneaten or mushy ones, so it's really a matter of finding the right balance of protein and fiber to keep them at a healthy weight and with healthy output(urine and poop) during their growing stage. It also helps to keep them used to eating timothy hay, as alfalfa hay fed exclusively, can make for a picky rabbit and make it difficult to wean them off of it and onto grass hay when they become adults and no longer need the extra protein from alfalfa.
One thing you do need to watch out for when feeding alfalfa, is excess calcium deposits in their urine. Seeing some chalkiness to the urine is normal, but if it gets thick, creamy, or sludgy, then it means your bun is prone to bladder sludge issues and needs dietary calcium reduced(so no alfalfa). But usually with young rabbits they need the extra calcium, so it's not usually an issue like it can be for adult rabbits.
I could be wrong, but I think it's around a year old that giant rabbits growth rate slows, so that would be about when I would start gradually removing alfalfa from their diet and feeding only a grass hay like timothy, as well as switching them over to a grass based food pellet.
