Using two would pretty much go against what the Basslink is stressing, which is minimal use of trunk space. Its pretty much designed to work alone. However, I personally did not like the Basslink when mounted in the trunk, it seemed to produce a faraway muffled bass. In hatchback though, it sounds great.
Yeah, if you get two - you'd get better sound out of 2 subs in a box with an amp or one really good sub and an amp....for the price that is - I am not accounting for space and all that.