Used henna, waited a couple days. Can I still use the indigo?

Hi! Okay, so I'm a newbie at this. But I bought a twin packet of henna and indigo from thehennaguys and it worked so well! I kinda liked the red so I kept it. Now I have an extra packet of indigo lying around. If the mood strikes, can I still apply it on top? Or does it need to be done right after an application of henna? Thank you!