Is using a single hanger plus a framing angle on a double member sufficient
No, it is lazy. A double joist hanger should be used for a double joist with few exceptions. Whether it is legal or not depends on how your state and local community have developed, adopted, and interpret building codes. From a structural standpoint it may be insignificant under certain conditions.
Is it worrisome: an engineer would be your best source but generally no if the structure is maintained and you inspect it regularly. Joist hangers are a component of a system and their absence or improper installation don't mean catastrophic failure by themselves. It is not uncommon in older structures to see double joists tied into single joists to avoid an obstacle. All without hangers.