Well in theory Raetia and Noricum are closer to Italy but I think the Illyrian region is far more important, if the Danube border is lot from the entire stretch from modern Budapest to Dobruja it looks to me that the remaining portions would be increasingly weak to supply using the river-based Roman system, it's not simply about where troops are at a specific point in time and after all I'm not sure the Danubian armies would be thrilled to move in Italy abandoning their territories to rampaging Goths, this looks like the groundworks for another proclaimed Danubian-Illyrian Emperor, which is pretty interesting in itself.Because I assumed Postumus would call all troops available from Dacia and Moesia to counter the invasion of 259, since as a usurper, his priority would be to defend Italy and the provinces closest to it. If he fails, the provinces in Eastern Europe will be the most exposed to subsequent attacks, while the Upper Danube would be a little closer to the Western Provinces, and be easier to aid.
No Roman emperor would be willingly down for that, especially considering whoever becomes emperor is supposedly a general. Optimistically, it’s more likely for the leaders to make an arrangement similar to the Tetrarchy, with each member holding a portion and all of them responding to the senior member.
Why would Roman Emperors find this arrangement bad? They would be indisputed superiors and have a way to deal with local defenses without resorting to elevate others as their equals, and it would also divide the resources that any give dux can have and leave to themselves the core areas of Italy and the lucrative and secure African province(s).
It would be similar to the Tetrarchy afterall.