When I suggested two No.3 injectors, I did consider of suggesting in having a smaller one for continuous running, but as 21C105 does not have an axle or cross-head pump I considered that two large injectors would be better.
On one of my locos I have this arrangement of large and small injectors, and on one running session both my cross head pump and large injector decided to fail at the same time and I found that small injector could not cope on its own very well.
Definitely agree about using a separate water supply for injectors.
Regards Luigi