I suppose this is a new control Zentyal added because too many users are setting IP on same subnet on 2 interfaces and then set one of this interface as external, which doesn't work.
But, even worst, if this was allowed and if you connected your 2 interfaces on 2 different networks, then machines on each network will not see each others unless you implement some very specific network tricks
![Wink ;)](https://forum.zentyal.org/Smileys/default/wink.gif)
Perhaps if you elaborate on what you intend to achieve this may make Zentyal to change their mind with this control
![Smiley :)](https://forum.zentyal.org/Smileys/default/smiley.gif)
For the time being, this is a pretty good idea
![Cool 8)](https://forum.zentyal.org/Smileys/default/cool.gif)