Well, the Hawthorne and Broadway bridges already have this sort of couplet. The paths on the Interstate Bridge are far narrower; four to five feet wide SB and 44" wide with a minimum of about 40" NB. If anything, it would make more sense to enforce what is essentially a couplet on this bridge as well. I already go with traffic, and would have to agree with you that most people tend to take the SB path. That's half of the reason that I take the NB path—nobody else seems to.
Besides the obviously needed improvements to the pedestrian and bike facilities and connections on the bridge, I think that enforcing a one-way couplet for cyclists would help make this bridge more crossable.
And if it matters at all to this discussion, I live in East Vancouver, usually taking the Glenn Jackson bridge via bike or bus Southbound and the Interstate Bridge by bike Northbound.
Gas Tax Holiday? I must be on gas tax retirement.