They two cannot appreciate at the same time because of so many reasons. I will talk about a few that I know about
Firstly, the level of a coin appreciation is dependent on the volume of transaction that the coin had during the time. secondly, it also depends on the volume of the coin that was mine during that time also.
It also depends on the fundamental analysis which is basically the news that the coin promoters are doing and the new projects that is built upon the projects.
I think with this few points, you should be able to understand why steem is appreciating along with bitcoin.
Thanks!