I really don't see the issue with it being played at the same time. The 2 sides are virtually completely different now, different coaches, only really Bairstow, Stokes and Root playing ODI and Tests. And Bairstow's replacement got a 100, Stokes wouldn't of played focusing on the new role and Root possibly rested as its a T20i world cup year not ODI world cup year.
Plus adding in the proper exposure it give an associate nation is always good for the game.