AD455 - the fall of the Roman west?

[Flavivs Aetivs]

The empire probably would have survived longer without Stilicho and Ricimer. Had say Constantius III been around at the time Stilicho became manager then it probably never would have come into the crisis situation it experienced in the first place.

The earlier practice of using several magistri would not have this effect - as far as I know, single supreme magistri were not appointed in the east either.

Not really, it usually resulted in one of them becoming the effective strongman like Aetius as well. Aspar and Zeno are prime examples. It eneded up being balanced more though.

Theodosius II tried to rectify this by appointing the ineffectual Felix as Magister Utriusque Militiae in 425, which instead resulted in civil war between him an Bonifatius and then Bonifatius and Aetius.

So could we say that the power of the magistri utriusque militiae in the west, rather than holding the empire together, might rather have contributed to its fall?

Killing them certainly did since beginning with Valentinian II and Arbogast the Empire essentially became reliant on a Child-Emperor system of governance, as argued by Meghan McEvoy.

I would say that the civil wars to obtain this position certainly contributed to the West's decline, creating a period of weakness for the Vandals to exploit thanks to subterfuge and open war between Felix and Boniface from 427-429.