I don't know how to configure so it will work like that, but I have suggestion
Get two static IP and assign one to each server
You can even do if only one NIC in the machine with virtual NIC assignments:
NODE1 interfaces
Code:
auto eth0
iface eth0 inet static
address 192.168.50.2
gateway 192.168.50.1
netmask 255.255.255.0
network 192.168.50.0
broadcast 192.168.50.255
auto eth0:0
iface eth0:0 inet static
address x.x.x.x
netmask z.z.z.z
NODE2 interfaces
Code:
auto eth0
iface eth0 inet static
address 192.168.50.7
gateway 192.168.50.1
netmask 255.255.255.0
network 192.168.50.0
broadcast 192.168.50.255
auto eth0:0
iface eth0:0 inet static
address y.y.y.y
netmask z.z.z.z
where
x.x.x.x is real-world IP #1
y.y.y.y is real-world IP #2
z.z.z.z is subnet-mask for these ip range
Then you adjust the nameserver records to point to the particular real-world IP of which server is hosting that particular website
eg:
myfirstdomain.com. A = x.x.x.x
www A = x.x.x.x
webmail A = x.x.x.x
firsthost.com. A = y.y.y.y
www A = y.y.y.y
secondhost.com. A = y.y.y.y
www A = y.y.y.y
That is how I have my servers setup currently (except they have quad physical NIC, so no virtual interfaces)