Swarmkeeper
Hero
I realize this may be a "reductio ad absurdum" argument, but would you ever prepare only Concentration spells? You must really like cantrips.
And yes, I'd consider scribing/preparing both fireball and lightning bolt. Different energy types, different AoE shapes. That AoE thing is especially true if you're an evoker that doesn't have to worry about frying their friends with their area spells. That archetype is easy mode when it comes to blasting.![]()
If I was a Paladin, my non-concentration spells would be quite limited - so, yes, I might have a situation where I prepared just concentration spells for the day. As a wizard, most likely not.
Just now, I was specifically looking at 4th through 8th level evocation spells for wizards (using www.dnd-spells.com) and didn't see any instances where I would rule out preparing multiple concentration spells. They all seem different enough.
Do you have specific examples of spells from your spellbook you would never prepare at the same time because they are both concentration spells from the same level? Or examples of two concentration spells you would only ever select one of when leveling up?