Add the series resistances before finding I = V / total R. Current is shared, but source power uses the whole source voltage: P = V x I. An individual resistor uses P = I x I x its own R. When voltage and total resistance both scale together, equal current does not isolate the effect of voltage alone. These steady ideal paper models do not predict temperature or component safety.
Worked example
Design X uses 4 V with 200 + 200 = 400 ohms: I = 0.01 A = 10 mA and source power is 0.04 W. Design Y uses 8 V with 400 + 400 = 800 ohms: I stays 10 mA, but power is 0.08 W. Both voltage and total resistance doubled, while source power doubled at unchanged current.
Three different source-and-resistor pairs
All switches are closed. The sources and constant resistors are ideal. Design A: source 5 V; resistors 250 ohms and 250 ohms. Design B: source 10 V; resistors 500 ohms and 500 ohms. Design C: source 15 V; resistors 750 ohms and 750 ohms.