Just as a quick note. With the code you have above you will only get the advert appearing once on each page which didn’t sound like what you asked for originally?
If you need an advert every two posts so if you had say 8 posts on a page you would end up with four or so adverts then you would need something like winslow4’s code to do this in the easiest way.
If of course you do just want an advert in place of the second post then your code will work. Note though that placing an :
{if count == "2"}
---Ad code here---
{/if}
code block inside a weblog tag will mean that whatever your second post content is won’t be shown unless you place it within the code block too.
Best wishes,
Mark