Advice - "multiDB" for multi networks but sharing common user tables in global db

If i run a wp/bp multisite and multinetwork, and i wanted all "sub networks" and blogs of those networks to run their own "db's" BUT i wanted all to share say the wp_users and wp_usersmeta tables from one common global db... which would be the best way to go about this? Multi-DB plugin? Preferably i would like each "network" to have its own db, but share common users and any related user info e.g. meta info and i guess buddypress tables (eg friends lists would present some issues"

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