"I am using it."
If you were instead to say "I give it to him," both the direct and indirect object pronouns could be used:
Je le lui donne / Je la lui donne
If you'd like to test yourself, tolearnfrench.com has one.
Edit: corrected example based on robertcolin's feedback by switching verb from utiliser to donner.
No, "Je le lui utilise / Je la lui utilise" isn't correct, Je l'utilise pour lui. And "l'" is contraction for "le" or "la" but that could be "I am using him (her)"