Like most guys, my favorite exercise involves pushing stuff. That is, training chest. Walk into any gym and you'll see the guys always on the bench press. I don't know why this is such an appealing exercise, but it is. Technically, this is chest training, but since most guys don't use proper technique, it's mostly an exercise in futility. And at the very least it puts a lot of strain on the shoulders. If it was done correctly, you would see a lot of guys with big, strong, chests. But mostly you see guys with painful shoulders, walking around with a stoop.
And even if your technique is excellent, doing pushing movements puts strain on your shoulders. You can't move your arms forward without using your shoulders! So, of course, you can't isolate your chest muscles completely, they will train along with shoulders. And that means that your shoulders need to be a strong and fresh as they can be on chest day. And that means, if you follow me, that any shoulder-specific exercises should be done as far away from chest day as possible. And that means the the day after, never the day before, or even two days before. By the time chest day rolls around again, your shoulders should not be sore. At all.
Another nice thing about training shoulders on the day after chest day is that it allows some stretching and warming up of the chest muscles without direct training. I had a "frozen shoulder" about six years ago, so I know how delicate shoulder muscles can be, and I stretch them a lot, especially before training them. And of course you can't stretch your shoulders without putting some stretch on your chest, which is a good thing.
Trying to do shoulder-specific exercises on the same day that you do chest-specific exercises is a big mistake. After a chest workout your shoulders have had a pretty tough workout, too. Give them 24 hours to do a bit of recovery, then hit them.