|192.168.10.0 with 255.255.254.0 will give you 510 hosts with a range of 192.168.10.0 - 192.168.11.255|
Supernetting requires no router.
Unless these are two different physical sites there is no failover using subnets. You would not need a router at all if this is the same physical site.
To answer the subject line web search Internet Connection Sharing. A xp workstation would need two nics.