I would like to cancel both my accounts on pay-pal so i canhave just one account but a forgot the password and username can pay-pal erase them both

If there's no balance or transaction with the forgotten account, you need not to worry, it'll automatically be deactivated after few months but your original/existing account will be functioning properly without any hindrance or obstruction. When an account is not active or transaction being done for several months, paypal will send you an email to your email address with the message describing about the deactivation of the account, if you don't take any action, it'll be deactivated.

It's better if you can at least remember the email address [Note: your username is your email address] of your account so that you can deactivate it yourself. But still you can try to close it using the "Forgotten your email address or password?" form.
To recover your email address and your your password, use this form...
Forgotten your email address? (you'll need to provide your name full name, zip code, phone number)
To reset your password use this form;
Forgotten your password? (you'll need to provide your email address)
After you retrieve your password or recover your email address, login to your paypal account, click "profile" and select "Close Account" from the options available, you'll be presented with a warning if you are sure or not, click yes and continue with it.

I hope you could understand my instructions, if you have problem with my instructions, please let me know.

Good luck.

Thanks for using FixYa.