How do I write a cover letter?

I want to apply for this assistant marketing position requiring all the usual stuff (tracking campaigns, planning events, etc.). The problem is I’ve only been at my last 2 jobs for 6 or 7 months.

One of the companies eventually closed. I read the writing on the wall and decided to leave before closed. I was working at the job I have now at the same time as the previous job, but there is no growth here and I want to leave.

Both these jobs were stop gaps for the career path I am trying to find.

How do I write this cover letter to say I have a very diverse background and not seem like I’m unreliable?