on MA5680T I configured L3 VPN service for two ONT, two PC connect to ONT HG810 can ping each other. One PC became a FTP server, other was a FTP client but two PC could not executed FTP session.
I changed MTU value on ethernet switch port bigger than 1507, for example 1550 KB, after that two PC could run FTP normally.
Run Ethereal software to packet header obtaining I knew that the FTP session was interrupted due to packet length was exceeded MTU value. the length of packet file after tag 2 VLANs is 1507 KB but MTU value on Ethernet switch port is 1500, by this reason the packet was dropped so two PC could not run FTP servive.