I like doing both for peace of mind. The inside one keeps the sewed seams from soaking up water and making the outside of the tent wet. The outside one is there in case you missed some and the moisture is seeping through. I tape mine with gorilla water resistant tape all the way around inside so it doesnt let water go under the tarp and sit, then get the seems wet. With the tape I haven't had any issues, its just to make me feel better. Especially since the tarps are so cheap.
Right now, I'm looking up non toxic plastic liners, shower etc..lol.And don't worry! I have set up 4 of these now so you can learn from my trial and error