Is it even possible to have UniUploader do this:

Take fileA and send it to SiteA under UsernameA (and passwordA), then take fileB, send it to SiteB under UsernameB/passwordB?

If so, how? I don't want to spam both sites with both files (and the accounts are different too).

That has been asked before recently. No, UU can't. You'll need to either send both user/pass combos and both files to both sites (only even possible if the post vars are different), or have two UU installs, or upload one automatically and the other manually.