[Samba] Questions about 2n dc and samba update

[Trenta sis]

Hi,

I have to upgrade 2 different samba AD DC (2 domains) from 4.4.5 to 4.10.7

First is a single server and updaded with in-place upgrade from 4.4.5
to 4.10.7 and all perfect

Second environment I need to add a second dc and also upgrade existing
server, and I have some question about how to do this:
- join second domain server with 4.10.7
- then make in place upgrade on existing server dc1?

reading wiki I'm not sure if rejoin is needed or not, as I have
described recently I have update an existing 4.4.5 (also fileserver)
with in-place method and all weems to work...

About second domain controller I have question bout dns config,
reading wiki amb not clear about what is the suggested
/etc/resolv.conf on both dc

dc1
dc1 ipdc1
dc2 ipdc2

dc2
dc2 ipdc2
dc1 ipdc1


or
dc2
dc1 ipdc1
dc2 ipdc2

dc1
dc2 ipdc2
dc1 ipdc1

Thanks!