The two cards will work fine together.
You'll want to make sure each network uses a different subnet;
e.g. 192.168.0.0/24 for the first and 192.168.1.0/24 for the second, or 10.x.x.x. etc. Just so long as the network part is different (e..g the bit masked by the 255s in the subnet mask).
You would then be in a situation where your machine doesn't know which to use for the internet, so, you could either manually configure your IP addresses, and leave out the default gateway on the interface that you *don't* want used for 'net, or, more properly, just set the Interface Metric (this means the 'cost' for using the interface). The lowest metric takes priority.
So set the NIC which is connected to the Sky ADSL to 2 and set the preferred one to 1.
I have to say I'm not sure if it would just work even if they were on the same subnet.. the machine might just send the ARP broadcasts out of both NICs and therefore still find a machine whether it's on NIC1 or NIC2, but for simplicity I'd say make sure they're on different subnets.