I think that biting them back just shows them that biting is OKwith you.
11 months is very young for them to understand. My daughter used to pull my hair, so as soon as she did I would put her down (I was carrying her) she soon stopped.
Maybe if you catch one biting move them to somewhere that they don’t like for a minute or two like into the buggy in the hall – away from all the fun. This is difficult to sort out when they are so young, also you don’t want them to think of the buggy/playpen/cot as a punishment.
Giving the other child lots of attention and ignoring the one who bit might work, but at that age they probably won’t know why they are not getting attention.
Sorry not much help, good luck