Let's assume we have a Western Caesar with Britannia-Gaul-Hispania, an Eastern Caesar with Thrace-Anatolia-Syria-Egypt-ELibya, and the Augustus holding what's left and senior to the other 2.
We've got a roughly stable succession of West>East>Centre by the 350s.
What happens when the Huns hit?
I assume the Foederati idea will still occur - where do these get settled?
Fully depends on the actions of the Caesars in the meantime. A well led Western Empire could fight alongside, stabilize or annex/integrate/assimilate the tribes that Attila displaced - which would render Attila vastly weaker, if poorly led, the West could be hard pressed on the Rhine, and relying on the Centre for support.
If well led, the East could defend against the Sassanids, likely with help from the Centre, but if poorly led, the Centre may have to give all its resources to supporting the East.
If well led, the Centre could grow vastly, and create a strong frontier (perhaps adding territory for strategic depth), If poorly led, the Empire could fragment.
Assuming all things good/average/better - the western Empire would likely be responsible for settling foederati. But if the foederati can be used to prevent Roman territory being depopulated in the first place, then they can stay beyond the borders. Which, IMO, is the best way to use them. Having an Emperor focusing on the West, well aware of his limited resources, but able to use his resources well, would probably find such a policy, even if partially fulfilled by the Centre, the best way to expand/defend his turf. Winter is coming, and if you can make winter less awful, you win the hearts and minds of a lot of tribes.
Regarding the Huns, if the Romans have shown that they're more than capable and willing to fight in Germany, etc
alongside the Foederati, then they become a less tempting meal, and are more agile at defending their European Borders.
They could well decide that the Sassanids were a better target than the larger, stronger, well-organised Romans regardless, and invade them in full force.
But assuming an invasion - probably fought off as per OTL. Perhaps easier with the border forces better led. Crucially, the Earthquake that threw the East into chaos can be ignored by the Centre - who can prevent the devastation of the Balkans.
Even earlier, if the Huns can be prevented from their invasions of the East and Centre in the first instance, which with more troops from the Centre in the Balkans, and the East fortifying Armenia against the Sassanids and others, could be done - I just don't know the numbers of Huns there were in those invasions.
TL;DR - all things going well, the Huns can be repelled without the worst of their devastation. Just don't let them gain non-Roman European Hegemony in the first place.