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
Perhaps if you elaborate on what you intend to achieve this may make Zentyal to change their mind with this control
For the time being, this is a pretty good idea