Which is better: Bitcoin's monopoly or dual power with ethereum?

Bitcoin's dominance has gradually declined in recent months and now stands at just over 40 percent. This enabled altcoins to rise in value. At the same time, ethereum's dominance has grown to 20 percent during this time. A little more and the dominance of these two coins can equalize.
 If this happens, how could this affect the cryptocurrency market, as well as bitcoin and ethereum? Will dual power in terms of dominance be more useful for cryptocurrencies than the current monopoly dominance of Bitcoin? How will altcoins behave if bitcoin and ethereum show conflicting price movements?