I think that if your intended meaning is "We'll watch the entire movie with you," it's best to say Просмотрим фильм с тобой. If you say посмотрим here, it means "We'll watch a little bit of the movie with you (and then we'll stop watching it)."

So, although по- makes the verb perfective and thus signifies that the action was completed instead of continuous, it also has the meaning of "just a little bit" or "only for a short while."

Compare:

Она читала книгу. = She was reading (and reading, and reading) the book. (imperfective)

Она прочитала книгу. = She finished the whole book, cover-to-cover. (perfective)

Она почитала книгу. = She read the book for 15 minutes or so, then stopped reading. (also perfective, but emphasizing the short duration of the activity)

P.S. Just for the sake of practice, yswaq, it's a good idea to make a habit of spelling out your numbers in Russian, instead of using numerals:

Вчера, Жанна покупала две сумки.

Мы с тобой будем смотреть два фильма (потом / позже).

Note that "фильм" is masculine but "сумка" is feminine. Also note that "потом" is variant way to express "a little while later" or "next, after this."