Example: You fly one airline almost exclusively for domestic and International travel. But sometimes fly someone else like an LCC to save a little money or are forced to take them if your main carrier doesn't get you there, so you "tag in" the other airline. It can be two legacies or a legacy and lcc.
For me I have flown SWA and AA. I think Ill stay with AA for certain domestic and all International Flying(or ONEWorld) and use SWA for some domestic routes like Vegas, JAN, or something else.