As long as your subs can take the combined re-directed bass in addition to the LFE, there is no problem with recombining the signals. There is a disadvantage to using a dedicated LFE sub however, and that is that the LFE will be only reproduced by one sub instead of two. I prefer to have the added oomph of having the LFE reproduced by more than one sub personally, in addition to the room mode averaging afforded by multiple subs.

The ICBM is a workable solution, however I think (I could be horribly wrong) that the stereo sub crossover ability is limited to a fixed 80Hz frequency.