Two power supplies with different current in series
I don't see a problem with your approach, looking at your description I think it would work just fine.

You are placing two voltage sources in series, which is alright.

The upper voltage source has a significant load connected to it only, which is relevant to these two items ONLY (note that the Thevenin equivalent to the two node circuit comprised by those two items, a voltage source and a resistive load, is the voltage source only). So that's fine too.

The opamp gets a suitable symmetrical supply, okay there.

There's still plenty that could be wrong as your equivalent diagram is very crude (an opamp is not exactly a resistor...), but what you've drawn there is alright.