Some opinion from me too...........
1) First of all, I would not tie this feat to BAB. I'd better just let the feat allow you to cast 2 spells (with casting time not more than 1 standard action) in the same round as a full-round action. Not possible to get more than 2, although you can use Quicken Spell to cast a third spell in the same round if you have the other feat as well. Replace the BAB requirement with simply (for example) caster level 10.
2) One restriction could be that the 2 spells must be the same (except eventually that they may have different metamagic applied). This eventually makes the feat works quite like Twin Spell (from T&B) with the following exceptions:
Code:
Twin Spell Speed Casting
action standard full-round
slots used one at +4 two at +0
metamagic yes no
The slot cost is balanced IMHO, TS let's you use a MEA in the same round but requires preparation since it's metamagic.
3) You may think the above is a too heavy restriction, but if you have in mind an artillery-like spellcasters it should be fine. You could add a second feat that lets you cast two different spells but of the same school, with this one as prerequisite.
4) Another extra restriction could be to require a Spellcraft check for each of the two spells so that it is more difficult to cast higher-level spells in this way.
A final note:
we have played for years with the 3.0 Haste. It was indeed too good for its low level, but you feat it won't be nearly as good, and therefore it should not overpower the caster unless you allow 3+ spells/round.