Summer Corrin or Morgan (F) or both

I was referring to any mix of units not specifically a "Flier" team if that's what you thought. It's just better to add other weapon types to help cover weaknesses that certain units might not be able to handle. For example IF you were to decide to go ahead and run these two units together and you happened to run into a team that focuses on using high Res units (many new armor teams are starting to look like this) than eliminating them with two blue tome units might prove difficult especially if the team is overloaded with green units. It is true you could run a red unit along with these two to help counter their weakness but what happens if that unit needs to kill 3 enemies. Can it one-shot all 3 enemies? Is it able to survive 3 consecutive encounters if it can't one shot them? This is why in most cases it is recommended to balance your team with different types of units. There's nothing wrong at all with running two blue units but maybe instead of running two blue tome units you could run one and a lance unit as well. This would help deal with some of those pesky high Res units that your tome user might not be able to kill.