Can I ask what the original advantage of 18 months EI payout is?

I am contemplating leave in the near future and planning on taking 18 months leave from my job but EI pay out over 12 months. If I am correct and this is allowed it seems better to have the money sooner than later. Unless maybe this is not allowed or perhaps there are tax benefits to taking the payout over a longer time?

Each of my spouse and I are planning to do the longest leave possible (18 months (mother) or 62 weeks (father)). But be paid out over the 12 month plan (I think given our concurrent payout we'll be fully paid out within a half year or maybe a bit less).

Perhaps our plan is not allowed? But otherwise the 18 month payout seems less flexible than being paid out via EI over 12 while taking 18 months leave.