How is it unbalancing? It's numerically identical to the current Ranger TWF class feature. It's a fluff change only. Kzach doesn't like dual-wielding two 'large' weapons, like two longswords, or two bastard swords. This change forces you to wield a light blade in your offhand (or one in your main hand too). The class feature lets you wield a longsword and short sword or two short swords with the same damage output as two longswords (as per the RAW class feature). The feat allows you to wield a bastard sword and short sword with the same damage output as two bastard swords.
Note to Kzach: why specify light blade? Why not off-hand weapons instead?