The cheapest and easiest way to achieve this would be to use subdomains. That way you can get a wildcard cert and use just one IP address.

For those still coming across this problem, check out this post about how to Run Multiple SSL Websites on IIS6 Using a Wildcard Certificate

As @CIA said, you can combine the loops by just running psexec twice in one loop.

$username = Read-Host "User name:"
$password = Read-Host "Password:"
$fullname = Read-Host "Full name:"
$computers = Get-Content C:\Tools\scripts\ServerList.txt

foreach ($computer in $computers) {
    psexec \\$computer -u username -p password net user $username $password /add /fullname:""$fullname"" /comment:"Comment"
    psexec \\$computer -u username -p password net localgroup Administrators     
}

But it seems like what you're really asking is how to run both net commands in a single psexec session.

$username = Read-Host "User name:"
$password = Read-Host "Password:"
$fullname = Read-Host "Full name:"
$computers = Get-Content C:\Tools\scripts\ServerList.txt

foreach ($computer in $computers) {
    psexec \\$computer -u username -p password net user $username $password ullname"" /comment:"Comment" ^&^& net localgroup Administrators     
}

You can try it this way, but I'm not sure it will work. The && is escaped using ^ so that it gets passed to psexec instead of being interpreted locally, which would have then run the second net command locally.

Alternative

What you could do instead is not use psexec at all and use PowerShell remoting. It just has to be enabled on each machine you want to remote into. I realize this might be a bit of an undertaking, but it will be worth it since it's way more versatile, and basically, it's the future.

If you're in a domain, you can even use Group Policy to enable PowerShell remoting (full disclosure: this is my article).

If you did that, your code would look like this:

$username = Read-Host "User name:"
$password = Read-Host "Password:"
$fullname = Read-Host "Full name:"
$computers = Get-Content C:\Tools\scripts\ServerList.txt

foreach ($computer in $computers) {
    Invoke-Command -ComputerName $computer -ArgumentList $username,$password,$fullname -ScriptBlock { param($u,$p,$f)
        # Everything in here is executed on the remote computer
        net user $u $p /add /fullname:""$f"" /comment:"Comment"
        net localgroup Administrators     
    }
}