The very first ever video game was a game called "Pong", basically it's a bit like tennis and is a great strating point for making a 2 player game in scratch. The following 4 videos will show you how to build it all.
|vid 1 - paddles and background|
|vid 2 - move the ball|
|vid 3 - hit the paddles|
|vid 4 - scoring and random elements|
If you find it easy to build then why not make it 4 player and have 2 paddles per team, a bit like table football