@Denis M Goddard yes. If you also set the port config as mentioned, then it can run at the same time as your main instance. If you also set the caps keys as mentioned, it exists in a separate/isolated network. (But note, as @mixmix mentioned in %gOTdlhx..., the sign key should not be changed mid-feed.)

For consistency I would suggest only use ssb_appname=ssb-test for testing/development purposes. Other identities/ssb-servers could use other appnames, e.g. ssb-foo.

There's also ssb-identities for the Node.js stack. That is meant to allow having/using multiple ssb identities in one ssb-server (without needing multiple copies of the database). I don't think I have seen any SSB clients using it though - except Feedless which uses a forked version.