The problem I have with saying just get two winter tires on the rear is that it just helps you do the one thing you shouldn't be doing if all-season tires aren't grippy enough in the winter, and that is go faster.

I always say do winter tires in sets of four, as braking/steering is more important than going fast in the winter, and most of that is done with the front wheels. Before my Dakota I had a RWD S10, and I put winter tires on it, and threw two 100lbs tractor weights in the back (and a box to hold them in place), and that thing was amazing in snow, and not just accelerating, but braking and turning as well.

Don't forget, if your winter tires on the rear are able to get you going to 100km/h, what is going to get you back to 0? The all-season tires on the front you already deemed weren't good enough to get you going?