Only once I have sat in FC and the Mrs in coach. I had flown from SAV->SLC and was meeting her at the airport so we could immediately fly to Costa Rica (vacation) on a red eye. As it was a red eye and we were sleeping it was okay.

Last year I gave up very valuable miles because we could only get one FC ticket - we used them for 2 coach tickets instead.

One other time I got an upgrade and paid $100 for my wife to sit with me. Way too much money but her dad had just died so it was worth it. On the second leg I gave my FC seat to my sister in law and sat in the very back with my nieces.

But that is me - if the arrangements work for them what do I care.