I'm pretty sure they can store spermatophores from many males... the males don't have a way of pulling out the spermatophores of previous mates, as far as I know, unlike some other animals: I know dragonfly males try to get previous mates' sperm out of the females before mating, for example.
Some squid have been reported to mate multiple times - there was a presentation at CIAC (2006) about Loligo bleekeri males with different behavioral strategies - larger males would implant large spermatophores in certain locations on the female, while smaller 'sneaker' males would implant smaller spermatophores in different locations, possibly to avoid detection by the larger males. And female Ocythoe have been found with jettisoned hectocotyli from three or four different males entwined in the gills.
Females octopuses can mate with several males. They have two oviducts, so they can even mate with two males at once! Sperm can be stored for up two ten months (probably longer) in vulgaris. Although I don't know of any published accounts of multiple paternity in octos, I'd bet it happens.
Do you know if a female can reject sperm once received? I saw what I think is unusual while watching my Mercatoris'. After her eggs hatched, my female was approached by her initial suitor and they mated again for over an hour (I was facinated and stayed to watch and photograph the entire time). Within two days, she left her den (the only one she ever had) and dissappeared. After 4 weeks, I still have not located her or her remains but do see a shell strangely positioned and have seen it change position once (at the 4 week mark) since I noticed it. My theory is that she thought she should brood eggs again (or that she did not want any more male attention ;>). I don't think she has eaten over this period and if the shell had not changed position, would feel certain that she had died some time ago (the shell is blocking the entrance to an opening in the LR but is positioned in a way that it would have to be wedged tightly or held in place and it has been flipped around once). If she could expell the sperm, I would have thought she would have and not have disappeared into a new den but thought I would ask for an opinion.