I think it would be a good idea, but the logistics would be a bit difficult to work out. My idea is that it works something like this.
The initial team can be chosen from the top 3 at the half marathon championships. Then, if those top 3 were to also be first at the marathon champs, the next 3 places get picked for the team. It would be unreasonable to expect someone to run both the marathon and half marathon at the same event. What do you guys think?