You don't say what kind of service the server is giving. Although not very fine-grained, you can sometimes get good-enough balancing by configuring the server with two distinct IP addresses as if they are two different servers:
Idea 1. on different DNS names (eg
Idea 2. alternatively as two A records for one name and let DNS's round-robin behaviour randomly allocate to clients.
My starting point would be to partition the switch into a top-side and a bototm-side. Best if you have proper modems (not routers) and you configure two Dialer interfaces on your router (via PPPoA). Do all DHCP on the router not the switch. Configure the server as if it's two servers.
Get router to send traffic according to which server its for.
Just to be clear: if it's resilience you're after, one goes about it a different way (two routers, two switches, two power distributions, only joining at unremovable single point of failure.)
Hope that helps.