I've never seen anyone talk about this before so I think it's better to ask for a straight answer, can Asic miners do double mining? I know its possible on Graphic cards but is it possible to mine two different coins on an Asic miner? For example BTC + BCH or BCD.
Each ASIC is tailored to mine a specific algorithm, so you will find them for a given algorithm. This means all ASICs meant for Bitcoin can mine all other cryptocurrencies that use algorithms similar to Bitcoin's.
ASIC miners optimized for BTC are usually pointed at whatever SHA-256 coin is the most profitable to mine at the moment.
Since BCH and BCD are forks of btc, they use similar algorithm of btc so you can mine them in an asic miner but not at the same time I believe.