Ok, now I understand better.
It is possible to use two power sources at one time but a little less safe - because the power distribution is less controlled. In case of any damage or short circuit, the consequences can be much higher, so you have to be careful. It also can increase EMC emission, so we can’t take responsibility of that
Also the fuse on the RPi is more loaded and you have to think about the maximum load on the 5V line. The CORE2 itself will draw maximum 0.2A on the 5V line, so it all depends on other circuits connected to the CORE2.
Motors and USB host can be used only with DC IN connected. It would be preferred to check the DC IN voltage in your program code and allow to use motors only if the DC IN is greater than ~7V.
My notes in the previous post are still valid, because powering RPi from its own USB is almost like connecting power supply to the internal 5V power line in CORE2. Not recommended, but I would not except any problems if the powerbank will stay longer than main DC IN power supply; or if two supplies work simultaneously.