I have done work before on the same tattoo, where lines are not fully healed. I stick far away from them and only hit skin i have not touched. But frankly its not worth it. It adds another session to the mix, and another two weeks to healing, in order to be able to hit the seam between.
It takes two weeks for the upper layers of skin to heal, and finish peeling. Frankly, the skin at this point is still very tender and fresh. The upper layers have healed, but the lower layers have not. You can tell this by the shiney, almost cloudy stage of the tattoo. If you tattoo at this point, its easier to overwork the skin. An overworked skin will bleed more, and have more risk of scaring. Scaring will have rough, raised points, and ideally you want your tattoo to be seamless with the skin.
Another thing to note, is certain effects and blends will be wayyyyyyy better if you can wet blend them in one session. Having to go back in after they have healed makes a bit of a harder transition depending on what your going for. Pigments when layered fresh will have a chance to mix within the skin. Pigments that have healed cannot mix, but will be layered over each other. Darker colors will always shine through lighter ones.
With all those things in mind, yes, it is possible. If you stick away from heal8ng areas. But no. Its not worth the risk of damage to skin, or quality of work.
That tattoo is a walking billboard of your work. Take pride in it. Baby it. And don't be afraid to tell your client no. People get excited to finish projects. But some don't understand the process the body goes through. Patience will give you the best tattoo. =)