How can I separate 2 power sources so only one can flow at a time?
As stated in the comments, only one relay with normally-closed terminals will do the job.







simulate this circuit &ndash; Schematic created using CircuitLab

Relay is a simple relay with 5V coil voltage. SRC1 or SRC2 can be either 5V/2.1A or 5V/1A sources (i.e. whichever you want).

The NC (normally-closed) terminal is connected to SRC2, and the NO (normally-open) terminal is connected to SRC1.

If SRC1 is not available, SRC2 will be applied to the load through NC terminal of the relay.

If SRC1 is available, the relay coil will be energized and thus the load will be supplied through "only" SRC1 even if SRC2 is available too.