In Ads Manager, I can easily create a custom report that aggregates information from two ad accounts. For instance, the "Total Reach" metric in a custom report accurately sums the number of unique Accounts Center accounts reached across BOTH ad accounts.

However, when attempting to achieve the same programmatically using the Marketing API, I encountered limitations. While I can retrieve total reach for a single ad account with a request like act_<ACCOUNT_ID>/insights?fields=reach&level=account, I haven't found a straightforward method to aggregate reach data from two separate ad accounts.

Simply summing the reach values from both accounts isn't feasible due to audience overlap. I believe there must be a method, as Ads Manager can accomplish this task seamlessly.