BuddyPress.org

This sounds doable theoretically. But i am sure there will be implications 🙂

abc.com – table prefix wpabc_
xyz.com – table prefix wpxyz_

Now in xyz.com, if you want to use buddypress tables of abc.com, go to active theme of xyz.com > functions.php and add the following code: ( untested )

add_filter( 'bp_core_get_table_prefix', 'bporg_244943_change_table_prefix', 99 );
function bporg_244943_change_table_prefix( $prefix ) {
    return 'wpabc_';
}