Neelm or Blue Sapphire as they say stands for Saturn/Shani, while Ruby stands for Sun/Surya/Ravi.
Saturn is considered cold and Sun as hot in astrology, at least, thus wearing both of these stones at the same time is considered to be contradictory, nullifying their advantages.
The above is the orthodox traditional view of astrologers all around.
My personal belief though, is that if you need to wear them at the same time for remedial purposes based on your personal horoscope, you can do so.
Good luck