Configure it 2 times as below.
set system services dhcp pool 192.168.1.0/24 name-server 4.2.2.2
set system services dhcp pool 192.168.1.0/24 name-server 8.8.8.8
[edit]
root# show system services dhcp
pool 192.168.1.0/24 {
name-server {
4.2.2.2;
8.8.8.8;
}
}
[edit]
root#