I would also second switching to Timothy pellets- gradually as to not upset their tummies from a diet change. That way you only have to buy one type of pellet- and the time seems to go by so quick from 9 weeks to switching to Timothy anyway. Like Sarita suggested- you could give some handfuls of alfalfa hay to your younger bunny to give her the extra boost for growing.
My bunny that I raised from a youngster was not thrilled about the alfalfa to timothy pellet switch (she had been getting grass hay all along)- and I did the switch over about a month period- gradually mixing in more Timothy pellet each week. But Cotton loves her pellets now that she has seem to have forgotten the richness of the alfalfa pellets. And both my bunnies go bonkers when they get hay that has some alfalfa mixed in. So it won’t hurt your older bunny to have a bit of alfalfa hay too if he happens to grab some.