ick. You wouldn't want to use .net and .com... I mean you could, but just no.
That's what subdomains are for. In your domain manager
You'd have whatever.example.com point to server 1, whatever2.example.com point to server 2.
You'd have www.example.com point to whichever server hosts your website.
You'd have example.com also point to whichever server hosts your website (so that people can get there via example.com or www.example.com, you'd probably want to setup a permanent redirect from example.com to www.example.com)
You'd have mx records setup to forward incoming mail to whichever server you want.
You'd have SMTP setup to send out mail from whichever server you want. phpmail() can be setup to use SMTP in your php configuration.