"Two sources of truth" is an anti pattern in computing and isn't redundancy. Use replication or backups for that. You can't imagine simpler than what you desc

"Two sources of truth" is an anti pattern in computing and isn't redundancy. Use replication or backups for that. You can't imagine simpler than what you described because it's single entry. Double-entry is twice as complicated, makes sense only to accountants and not to computing people. Your example of 1 transaction would be doubly-kept as some nonsense like BucketA revenue:$x CR cash:$x CR BucketB revenue:$x DB cash:$x DB