It would definitely be unbalanced.
Physical modifiers cost twice as much as NEGATIVE mental modifiers, but positive mental modifiers are priceless. A no-ECL race with +2 INT becomes an instant Wizard-magnet, no matter what physical penalties you give it. It's one of the reasons that the designers specifically avoided putting races with mental boosts into the core rules.
It's just that having a penalty to a mental stat you don't use isn't very crippling; as much as Half-Orcs have been bad-mouthed, the INT and CHA losses never really hurt the majority of their classes, while the STR boost nearly always helps. While most classes gain from a physical boost, they need all three physical stats. You can't just ignore DEX and CON to raise your STR. On the other hand, the few classes that need mental stats really only need one mental stat. A Wizard can get by with lousy stats all around if he has a huge INT. Having some DEX and CON is nice, but STR, WIS, and CHA can pretty much be ignored.
So, giving a Wizard twice as many points to put into INT is a huge power swing. Since casters are pretty much the only ones who make raising mental stats a high priority, the only ones who would benefit from your proposed 2:1 ratio are the ones who are already benefitting most from just the old-fashioned +1.