wanna do both with only one board
Stand up paddlesurfing and
longboarding

l do not think you can man. Paddleboards are much wider and thicker than a regular log.

No, you would need a 10'-12' footer.

The Surfboard Man
http://thesurfboardman.com/

If the board is really wide then maybe. Most paddle boards are like 30 inches wide, my long board is only like 23 inches.

not really. you cant stand up on it when youre not going fast because paddle surfboards are a lot wider

One is not as good for the other. the paddle board is wide with a thick rail to give you an easier time balancing cause you are working hard standing up on the thing.
Longboards are not going to be as wide cause it makes it hard to paddle when you are lying down or on your knees. and ususally the rail is not as think cause you are going to have a much harder time turning without the paddle.
Wish it were more cost effective.
l feel your pain.