Fatal!Error has been described by Xedramon as a Geno who has lost everything, including his own mind (meaning he is an insane depressed character like Error!404)
The only thing he seems to care about is his brotherly love for Papyrus, He now travels across universes trying to collect codes to "sew" together with a new Papyrus. But, his attempts continue to fail to only create broken versions (as the codes he collects are from other Sans' since he doesn't want to hurt any Papyrus from other universes).
So i'm pretty sure if Fatal!Error were to meet Error!404, and since Error!404 is basically a Sans, Fatal!Error would try his best to brutally murder and take Error!404's code but since Error!404 is stronger in any way possible he'll just kill Fatal!Error. but IF they fused I think their name would be U.N.K.I.L.L.A.B.L.E "In my opinion, the name just fits to the fact that Fatal!Error can't die and Error!404 being one of the strongest Gods.