Actually, it appears that CSX and NS get along generally well, especially when such behavior is considered to be mutually beneficial.
NS and CSX have worked out a lot of "shared" trackage rights, so that time and $$ can be saved in trip times, reduced track maintenance costs, etc. So, if there is a $$ and sense reason for doing something, they'll probably try it!
As far as trading units, they usually don't trade units. It's like Charlie described: Other railroads hand over the power on through trains to CSX, regardless of who it belongs to. Sometimes NS, other times UP, BNSF, CP, CN, KCS, TFM, etc.
Such varied locomotive "scenery" makes train-watching (and life) very interesting!
You're probably too young to be aware of the movie "Forrest Gump". In the movie, he said "Life is like a box of chocolates: You just never know what you'll get".
So is train-watching, when you never know what kind of (or whose) locomotive will come around the bend. I hate it when it is predicable!
Also, a railroad may end up with a NS unit on a run-thru that terminated on their line. Often, in such circumstances, they'll assign the NS unit to a train that serves as a means of getting it back "home", such as on a train to CSX, so that CSX can put it on a CSX freight headed to a location where NS crews can drop by and pick the unit up.
I see a lot on NS units on CSX freight trains in south Georgia and Florida, sometimes as the lead unit! Now that's a sight to see!
Catch you later!
ACLfan