You need NickServ account on any server.
1. Open clean (mirc.ini) mIRC.
2. Use: /server -fopu serverwhereyougotNickServ port -i NickHere AnyOtherNick something something -l msg login password
3. Now you are connected.
4. In Treebar you see connected network name and yours nickname (NickHere above).
5. Use /server -n.
6. Now you see two servers and first is connected.
7. Use /nick AnyOtherNick on 2nd server and for 2nd server nick is not change on Treebar - but it should be.
Correct is:
1. Open clean (mirc.ini) mIRC.
2. /nick a
3. /server -n
4. /nick b
I can try even simpler
