The problem with allowing a caster to take one feat to get +2 to both Spellcraft and Concentration is that most spellcasters, especially wizards, will take it. "Must-have feats" are, IMO, a bad thing. They cost feat slots a character could be using to make him or herself more unique and special.
Not all rogues take Stealthy because not all rogues focus on sneaking around unnoticed. Rogues can focus on getting into difficult places that the rest of the party can't reach (Climb and Balance), finding and disabling traps (Search and Disable Device), robbery (Pick Pockets and Open Locks), and a whole host of other activities. If all the rogues in your campaign take Stealthy, then that probably says more about the challenges players face in your campaign than about game balance in general.
Spellcasters, by contrast, mostly cast spells. Most of the spellcaster's choices come in the choice of spells, not the skills needed to learn and cast them. This is especially true for arcane spellcasters, who tend to have fewer class abilities than divine spellcasters. Arcane spellcasters also rely more heavily on Concentration because of their lack of armor, and wizards rely more heavily on Spellcraft to learn new spells.
As a final aside, Concentration and Spellcraft aren't IMO truly related to one another in the same way that Move Silently and Hide are; Move Silently and Hide are both integral parts of a single activity, sneaking. (And as mentioned, both use the same key ability.) Concentration and Spellcraft are both helpful to the same class of person, but the skills themselves are very different and have little to do with each other. What does the ability to ignore pain and distractions have to do with being very knowledgeable about how magic works? Not much. The two skills aren't even used for the same activity; you use Concentration to actually cast the spells, but you use Spellcraft to learn them.