Fantastic work on getting to chapter 19 without pairing up anyone!
You gotta pair-up every unit that is not a healer or rallybot. Your units will survive ALOT longer paired up than going solo. There is literally NO benefit to going solo rather than pair-up.
Its best to pair-up the same couple all the time to raise their support level and hence raise their dual strike/guard chances and pair-up bonuses.
Ha, I'm a clown. I assumed there must have been some penalty like reduced exp or something so I tried it literally once on the tutorial you learn about it and never again, I'll give it a go later.
Good to know, cheers!