USB and UART simultaneous power to Arduino?
That's perfectly fine.  As it's the same 5V source the voltage should be the same and it'll just combine them together as if you had 2 wires in parallel.  However, if there are things like diodes in the path of one connection that causes the voltage to be slightly lower then the increased voltage imposed on the lower voltage side would reverse bias those diodes and end up cutting off that one source, so only one would be used.

So no, it's nothing to concern yourself over.