I am bonding with two young joey brothers. If these two have had previous owners, it can take a bit longer (a few months sometimes) to bond with them. To discourage nibbling (aggressive or "hmmm is this food?") type of biting I've been told that sour apple or other safe sprays that are used to keep pets from chewing will work. The owners of my little ones suggest you just avoid using it on your fingers and clean your hands with the same soup each time (And just before) handling them. This way they don't relate your hands (or other parts of your body) to food. They don't hate you...remember they are just trying to figure out who you are. Trust with these little ones can be tricky. Someone online says "It's a process, not an event" Keep up the good work and if one is related to the other, the stubborn one will eventually come round. Hope this helps. My little guy also got a bad habit of nibbling me from me offering them food through the cage bars. They will relate your finger tips to getting food each time. Be sure to avoid that habit :)
Have fun and best of luck!
Amy