ectopagus

ec·top·a·gus

(ek-top'ă-gŭs),
Conjoined twins in which the bodies are joined laterally. See: conjoined twins.
[ecto- + G. pagos, something fixed]

ectopagus

(ĕk-tŏp′ă-gŭs) [″ + pagos, something fixed]
An abnormal fetus consisting of twins fused at the thorax.