Wallet: how to run several bitcoind nodes on a server?

All you need to do is update the bitcoin.conf for the second port to point to a different datadir, and use a different RPC port.

Keep in mind that this will result in double the use of resources for everything. Also, depending on your VPS, running multiple nodes on a single machine might not be feasible.

In general, several applications should be able to use a single node and differentiate addresses, etc. at the application level.