Unable to run bird and bird6 at the same time

Dear all, I was trying to implement ipv6 on my route server, but I can not start two bird processes at the same time. I just compiled 2.0.7 twice, I mean , the first one for ipv4 , and second one enabling Ipv6 flang ( ./configure --enable-ipv6) and copy bird to bird6. I just can run one, bird or bird6, but not both . Always get a error message "bird: I found another BIRD running."