One for personal/business contacts, and the other for anything that requires registration or sign-up (forums, subscriber content, etc). This way, if your second account gets buried with spam, various advertisements, or nag emails, your personal account should be relatively untouched.
Well, if you follow my previous tip, then you can use one gmail account and GUARANTEE no spam.
To refresh your memory, gmail with the plus sign gives you fully protected email. Create your gmail account and then a list of random codes. For each use of the email, assign one of your codes after the plus sign. Then, create that label and filter in GMail. Then, the only thing that will get into that FILTERED category will be valid email. Everything else can be nuked!