Ethiopia claimed, and sort of controlled, Ogaden since around 1895-97 IIRC, around the time of Adua. I hardly see any European power that could possibly be interested in challenging that claim at the time, not even Italy. Italy may be interested later, but any scenario including Italian italian aggression on Ethiopia and subsequent defeat in alt-WWII (supposing nothing happening in the Horn has consequences so deep to impact on both World Wars, that is plausible but far from certain) would likely see the preceding Ethiopian claims enforced.
So, one way could be just having all, or almost all, of Somalia under a single colonial power. Italy is distintictly unfit to this task, since she mostly lacked the drive to push her control, or even her claims, inland in the appropriate timeframe, and however sha had not the economic strength and political will to enforce such a policy even if she wanted to.
Maybe a German Somalia can do the trick?
Another possibility is avoiding the coalescence of the Salomonid Ethiopian Empire under Johannes and (especially) Menelik. If the major Neguses and Rases of the plateau are kept divided under a nominal Negus Nagast at best, Ethiopia would never control Western Somali lands and would also offer a much lesser resistance to any attempt at colonial conquest. Even Italy can manage to take the whole place then. The aftermath is anybody's guess, since no Adua analog MAY have far reaching implication in Europe, so that butterfly storms are possible.