You cant split the host messages. The only three options are:
1. leave as it is and you'll get use to it.
2. Turn off ALL host messages kills landed ship track changes, base captures, win the war messages everything.
3. Right click on the message tab, click "add a tab" and put the host messages on a seperate tab and/or window.
I use the third option and have the message windows low on the screen. 5 lines for the regular text, and 3 lines for the host messages. Works great for me.