Well it depends on what is in the $form_block.
Basically you need to have a way of telling when to move to the next line.
From your example I guess $form_block prints one line, so it needs to be changed.
You need to start a line, then start the loop.
Inside the loop you need to check how many blocks you have printed.
if you have printed two blocks, end the line and start a new one.
Don't forget to end the line when the loop is finished.