As far as I can tell, a planet can give 5% of its income to a regional government. If it is in two RGs, it will give to just one, I think. Because it isn't in the budgetary expenses of those planets, so it must be an invisible item.
I think having a few RGs is a net positive, but there is no extra gain for overlapping RGs. Moreover, my outlying planets don't need cash as much as my home planets. Outlying planets have excess cash from the scrapping of subspacers to transfer population, and can afford to run deficits, especially as their upgrades are relatively cheaper.
