As far as I can find the descriptions it appears that they're the same. Why would 2 different IPA characters used then?
Also note that even if they were the exact same thing, ʎ is a single symbol while l̠ʲ is a symbol with two diacritics; if a phoneme is common or high-profile enough, it will often get its own simplex symbol, even if it could already be represented by some combination of diacritics before.
By way of extreme example, remember that the IPA has diacritics for indicating voice (◌̬) and voicelessness (◌̥); there's no real reason /t/ couldn't be rendered as /d̥/, or /d/ as /t̬/.
/t/ resp. /d/ are obviously important enough that they get their own symbol nonetheless, and the redundancy doesn't bother anyone.