Open Access to both Networks?

Now that the merger is almost a done deal, will/when will T-Mobile customers get access to the existing Sprint network? I know the New T-Mobile has a plan to merge the cell towers and use the Sprint Spectrum, but that could take 3 years. Will they open both Networks to share with both existing customer bases? I know Sprint Customers can already roam on T-Mobile but shouldn't they open the networks so our phones choose the best LTE band until the integration is complete?