Guanosine and adenosine are important neuromodulators in the brain and work in cooperation to mitigate the effects of stroke, traumatic injury, and other neurological events. Both purines can act on slow (minutes to hours) and rapid (milliseconds to seconds) time scales. A guanosine-adenosine interaction has been proposed in which guanosine modulates adenosine levels, and the two work together to control glutamate neurotransmission. Traditional methods to codetect purines, such as HPLC with microdialysis, are robust but lack the temporal resolution necessary to quantify release in real time. Fast-scan cyclic voltammetry (FSCV) has been used to detect guanosine and adenosine independently, but codetection has not been possible. Here, we developed a novel "scalene waveform" to codetect guanosine and adenosine with nanomolar limits of detection in real time with FSCV. The scalene waveform uses a slow rate (100 V/s) on the forward scan and the conventional rate (400 V/s) on the back scan; potentials go from -0.4 to 1.45 V and back to -0.4 V. The scan rates were optimized to increase the separation of the oxidative peaks for guanosine and adenosine. The temporal separation of the primary peaks was increased (4.6 ± 0.1)-fold at the scalene waveform compared to the traditional waveform. Both exogenously applied guanosine and adenosine and endogenous transient release were detected at the scalene waveform in rat-brain slices. We show the first method for codetecting guanosine and adenosine using FSCV, which can be used to study the guanosine-adenosine interaction and better understand their cooperative therapeutic effects.