When you order, you can specify extra things you will need. For example, you can ask for a secondary NIC on each server, and to connect up the secondary NICs through a dedicated switch. There are some caveats though:
* more expensive. When your servers have non-standard configurations, Hetzner charges a fixed extra per-server, per-month fee ("Flexipack"). The fees add up.
* The servers need to be in the same rack. If you order them all at once it's fine. But if you order, say, four servers now, and the fifth server a month later, the fifth one will go on a separate rack and won't get the private access to the four.
* Setup takes longer. If you order standard configuration, it will be ready in minutes/hours. When I ordered the extra NICs and switch, it took about a week to complete the order.
* A bit more configuration required. Primary NICs work out of the box. For the secondary NICs to work you need to tweak /etc/network/interfaces and the routing table.