No that is not what the -allcoins 1 command does. What it means is that during every single dev mining session you will need to load new DAG to mine for Claymore and after the dev miner is done, it will reload the older DAGs to continue your regular mining.

I know this for a fact because it was very difficult to mine small dagger alts with Claymore. Basically if you got an undervolted GPU, usually after the 5th or 10th DAG loading, one of the GPUs would hang.

So its definately not loading both ETH and ETC.