Genetically, the children would very likely be indistinguishable from being siblings.

Sine the fathers have nearly identical dna, and the mothers do too, the kids of one pair would be indistinguishable in terms of which mother/father they came from.

Youd only be able to tell if any of the parents had any unique genetic mutations that happens post split (which is very likely), but then also that that genetic mutation also got passed down to their kids, which is roughly 50/50

This is actually a known phenomina, that if one of your parents is an identical twin, your aunt/uncle (the other half of the twin) would also pass a maternity/paternity test typically.

Cublings

Could also do wife swap days to see if there is a difference.

No one died a horrible gruesome death and there is no danger of deadly mutant virus with 110% mortality rate, nothing for Walter to do.

Not really "odd". Just the basic definition of twins, which can be either monozygotic (identical) or dizygotic (fraternal).