Multivoice( Savage Species, p. 37)
If you have two or more heads, you can cast more spells than usual in a round.
As a full-round action, you can cast one additional spell (or use a spell-like ability in combination with a spell or another spell-like ability) each round. Each spell used in this way must have a casting time of 1 action; a spell-like ability must also take no more than a standard action to use. Using this feat provokes an attack of opportunity. If your concentration is interrupted during this simultaneous casting or spell-like ability use, whether by taking damage or by some other means, a Concentration check is required for each spell or spell-like ability. Failing either Concentration check negates both spells or spelllike abilities. You can also use one head to cast a spell or use a spell-like ability while another head activates a head-based special attack, such as a breath weapon or an eye ray.
You still cannot exceed the limit of one quickened spell per round (assuming you also have the Quicken Spell feat) when using this feat.
Without this feat, you can cast only one spell per round and cannot combine spellcasting with other actions.