Since childhood I've know it as:

Oh, McTavish is dead and his brother don't know it.
His brother is dead and McTavish don't know it.
They're both of them dead and they're in the same bed,
But neither one knows that the other is dead.

"Bed" in line 3 is rendered as a long, drawn out "beeeeeeeeeeeeeeeed" for as long as breath allows. This, in addition to adding comic effect, mercifully ensures that the song doesn't go on for ever - as the singers begin to pass out from asphyxia