You cannot create two hard disks in one command. You will have to use at least two commands. In PowerShell 5.1 Release 2 the New-HardDisk cmdlet has a CapacityGB parameter. So you can use:
$vm=Get-VM-Name$displayname$vm | New-HardDisk-CapacityGB10$vm | New-HardDisk-CapacityGB5