Websites under the same account can definitely be placed on different servers. Subdomains can be configured as separate websites as they can with other panels. If they are your websites you would do this by using the "advanced" dropdown during website creation and selecting the appropriate server. If they are customer websites you would need to set the customer up with multiple subscriptions to plans which provision to specific server groups like "apache group", "wordpress group", etc.
It's not possible for aliases/addon domains/subdomains configured under the "domains" tab within a single website to be on different servers since these all run within the same application container.