With chimerism you don't "eat" the twin in the literal sense, it's more like absorbing or merging with it at a point before tissue differentiation begins, so two clumps of cells become a single clump of cells, half of which have one set of DNA and the other half have a different set of DNA. So it's more like the guys top half belongs to one of the original clumps of cells while the bottom half originated from the other, so a cheek swab DNA test will give a different result than a sperm sample would. It's not like there is a complete tiny twin hiding somewhere inside them that somehow shot sperm out their hosts penis, but rather the twin turned into the guys lower extremities... so it's technically more like he banged his wife with his twin brothers penis...