If they are kept cool (35F-40F), in the dark, in a relative humidity of between 50% and 90%, they will keep for months. Under ideal conditions, potatoes will keep between 8-10 months. Keeping ideal conditions in the summer, however, is quite difficult as things tend to get too warm.

If you can keep them close to the ideal conditions, then they really should last until June. A few weeks before your new grow bags arrive, move the potatoes into a cool spot with indirect light, like a north facing porch. By the time you are ready to plant, they will have started their sprouting.

Why bother with planting twice? Potatoe can store easily for 6 months and more. I have some from my July 09. That is 7 months.
So I would say go ahead and plant them all at the same time. Potatoes are not tomatoes to plant for early, mid, late harvest, because they will store, unlike tomatoes or cucumbers.