Two physical interfaces in the same VLAN

ciscoasa(config-if)# interface TenGigabitEthernet0/7.20
ciscoasa(config-subif)# vlan 10
ciscoasa(config-subif)# interface TenGigabitEthernet0/8.20
ciscoasa(config-subif)# vlan 10
ERROR: VLAN 10 has been assigned to another interface