set interfaces bridge br10 member interface eth1 set interfaces bridge br20 member interface eth1 commit
Checking the interface assignment afterwards shows
# ip link show dev br20 242: br20: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000 link/ether 62:34:3a:8a:fe:49 brd ff:ff:ff:ff:ff:ff [edit] # ip link show dev eth1 3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel master br20 state UP mode DEFAULT group default qlen 1000 link/ether 00:50:56:b3:cd:ba brd ff:ff:ff:ff:ff:ff altname enp0s19 altname ens19
The later addition wins and the CLI reports eth1 is assigned to br20 master br20. A member interface can not be used multiple times.