If you are in one coven, you cannot join another coven until you leave your current coven (under that particular account).
You would want to click on "Change Status" on the front page of your current coven, then click "Leave this coven". After this you will be removed from your current coven, and now can apply to any other coven.
However, many coven leaders keep track of where their members have been, and if you have evidence of coven hopping (running from coven to coven in short time periods), they will be less likely to accept you into the coven. They may also speak with your former coven leaders to find out if you were kicked out, or willingly left.
Personally, I have problems staying in covens I do not like or cannot handle being a member of, and leave shortly after realizing that. When I leave, I tend to wait at least 2 weeks until joining a new coven, unless I've been specifically invited within those 2 weeks.