During a prolonged single-engine taxi, I noticed that the running engine (engine 2) showed 0.00 for fuel used after running for nearly half an hour. When I started the #1 engine, the fuel used indicators on both engines almost immediately jumped to 0.01.
Further testing showed that with engine 2 running at TOGA and engine 1 shut down, the fuel used quantity on the running engine did not increase from 0.00 even after running at this power setting for 5 minutes. When engine 1 was started, both fuel used indicators jumped to 0.01 again. With both engines running, engine 1 was pushed to TOGA with engine 2 at idle, and both indicators increased at the same rate, even with such a massive difference in fuel flow.
The expected behavior is that each indicator is based on the flow flow for that respective engine, rather than being tied to engine 1.