For each post, the agent picks the eligible connected partner with the highest Moz Domain Authority.
You do not pick the host store for each post. When an agent runs, Backlinker looks at your accepted connections and keeps only the ones that are still eligible under both stores' current settings: participation, DA range, topic fit, categories, regions, language, competitor policy and link limits.
Among the eligible partners, the post goes to the one with the highest Moz Domain Authority, because it passes the strongest backlink.
Why posts can go to the same store
If one partner keeps the highest DA, it keeps receiving your posts. To spread posts across more stores, connect with more partners in a similar DA range, or adjust your Get links settings (for example the Maximum host Moz DA) so that other partners become the best match.