Microfluidics minimize the amounts of reagents and generate less waste. While microdevices are commonly single-sided, producing a substrate with microchannels on multiple surfaces would increase their usefulness. Herein, a polymethymethacrylate substrate incorporating microchannel structures on two sides was sandwiched between two polydimethylsiloxane sheets to create a multi-analysis device, which was used for the spectrophotometric analysis of the ferrous ion (Fe²⁺) and the ferric ion (Fe³⁺), by utilizing colorimetric detection. To monitor the signals from both channel networks, dual optical sensors were integrated into the system. The linear ranges for Fe²⁺ and Fe³⁺ analyses were 0.1 – 20 mg L⁻¹ (R² = 0.9988) and 1.0 – 40 mg L⁻¹ (R² = 0.9974), respectively. The detection limits for Fe²⁺ and Fe³⁺ were 0.1 and 0.5 mg L⁻¹, respectively. The percent recoveries of Fe²⁺ and Fe³⁺ were 93.5 – 104.3 with an RSD < 8%. The microdevice demonstrated capabilities for simultaneous analysis, low waste generation (7.2 mL h⁻¹), and high sample throughput (180 h⁻¹), making it ideal for greener analytical chemistry applications.