The saint on the left is Cosmas: note the letter C next to his
left shoulder. Damian is on the right, where we can see only the
letters A and NUS of his name. Note that the sign of martyrdom in
this era is not the palm, as it will be later, but the crown, as
we see for example in the
processions of martyrs in the nave of Sant'Apollinare Nuovo.
This image in full
More of Saints Cosmas and