That's a very sensible and logical idea which is exactly why no one in Ireland of that time would have agreed to it!
You have to remember that the Unionists were opposed to Home Rule on principle, they even voted against the 1920 Government of Ireland Act that was the subsequent basis for Stormont. Unionist opposition to HR was based on a number of factors, most obviously there was the fear of "Rome Rule" and being subsumed into a culture that they found completely alien, this wasn't just paranoia, there was an increasingly influential element in Irish Nationalism that followed the ideas of people like
David Moran who wrote you had to be both Gaelic and Catholic in order to be "truly" Irish, not that the Unionists were any exemplars of tolerance but this is something that Irish Nationalists to this day have never been able to grasp.
The other big reason why the Unionists were so hostile to HR was economic, they feared loss of access to Imperial markets through tariffs that the Irish Parliament would impose. The IPP just saw Ulster as a cash cow and if it wasn't included in the Parliament's jurisdiction then there was no point having it. So Unionists saw little to gain from Home Rule.
Perhaps if William O'Brien's
All for Ireland League who favoured a more conciliatory attitude to Unionist concerns had been dominant in Nationalism then something like this could have agreed.