How did they sawn asunder?

Sawn is a past tense version of "to saw." As in to use a saw to cut something. Asunder means blown or spread apart. Sawn Asunder therefore means, cut apart.

Cut apart or in half with a saw

Honey! Your case of infidelity against me and your related angst goes completely asunder once you realize that last week, my pee-pee was castrated! Thus, I could not have cheated on you with that...

Past tense of the word saw. The board was sawn in two.

Although frequently referred to as sawdust it may be called swarf, although this term is more often used in connection with metal waste.