Make 2 balls move at once

I have been stuck with this bug for a long time. I need to make 2 balls move at once. When I add the second ball, both of the balls get messed up. Here is the code in my .m file.

-(void) onTimer {

ball.center = CGPointMake(ball.center.x+pos.x,ball.center.y+pos.y);

if(ball.center.x > 295 || ball.center.x < 10)

pos.x = -pos.x;

if(ball.center.y > 435 || ball.center.y < 35)

pos.y = -pos.y;

ball2.center = CGPointMake(ball2.center.x+pos.x,ball2.center.y+pos.y);

if(ball2.center.x > 295 || ball2.center.x < 10)

pos.x = -pos.x;

if(ball2.center.y > 435 || ball2.center.y < 35)

pos.y = -pos.y;

[selfcollision];

}

- (void)viewDidLoad

{

[[NSTimerscheduledTimerWithTimeInterval:0.04target:selfselector:@selector(onTimer) userInfo:nilrepeats:YES] retain];

[superviewDidLoad];

}

Any help would be great.