dual audio bluray does not lower audio or video quality in any way. there's enough space on the disc for multiple languages. its a different matter altogether if the mastering was done in a studio by people not well versed in mastering bluray movies or who dont care for quality.
Blu ray has enough space to include dual audio.
However as 6pack mentioned, quality of recordings is what really matters. You can't do anything if the recordings are crappy itself.
You need proper equipment (such as an AVR or blu-ray receivers with DTS HD-MA and Dolby Tru-HD decoding capability) to utilise those sound streams unlike AC3 or DTS that can be decoded by a vast majority of speaker systems.
IINM it is this high-def. sound stream that takes up most of the space in the blu-ray disc.