thanks hellcat i've been looking for that info for a while now. its terrible how on both the 1st and 2nd day the casualties were near antietams losses. just looking at those numbers you know Lee knew it was an all or nothing fight. i think somehow he knew it was his last chance to end the war. And you have to give it to the yankees too. taking the punishment they took and holding on... i think they knew it also.
of those that did, most of their casualties occurred on one day or the other - not many units were heavlily engaged on multiple days. The Iron brigade is an example - they also fought on culps hill, but most of their casualties were on July 1 west of town.
some units, such as the Union 12th corps fought over 2 days but in pretty much the same spot (culps hill).
Confederate units, especially those of Pender and Heth who fought on both July 1 and 3rd in Longstreets assault, are trickier but many have pretty fair accounting between the 2 days.
this type of question reveals an interesting tidbit. Think of a unit in, say, Hood's or Pickett's division: they march to the battlefield, march into position, attack for 90 minutes, lose 1/3 to 1/2 of their number and then return to Virginia.
brings to mind the saying of war being months of tedious boredom followed by an hour of total terror...