Just one will do, if you put too much glue on (by putting it on both sides) it will be more difficult to get them clamped together properly and may take longer to dry.

I usually do both surfaces, MDF can be so absorbent that it dries in patches before you can clamp up. If you have a large surface to do use a big brush 'cause you need to be quick.

greg even at that size it can be hard to clamp up evenly over the whole board, unless you have a vacuum press that is?

I usually put a few screws in where it is awkward to clamp, then take them out once the glue has set

and make sure it is absolutely flat as it dries