I love prep sinks in a kitchen. There must be room for at least a 12x12 under mount sink and a prep faucet. I would also and probably most importantly , think about the "flow" and if there is truly room for two in the kitchen.

If you posted pictures, we may be able to help you more. Need to see space.

It also depends on the size of the island, you don't really want to jam in too many appliances / fixtures thus making the island not as usable.