https://www.flyertalk.com/forum/34039508-post2033.html

When you call in to link, they add LKRL keywords to the PNRs.

This lets the system know you're together. If an agent pulls up one PNR, they'll see both. If you try to check in online, you'll be checking in for both.

When you divide a PNR, you don't get LKRL, but the PNRs are intrinsically linked (since they were, at one point, one PNR). And you get all the same benefits.

So you do not need to link two PNRs after dividing them.