How to plot ROC Curve for a cascade verification system?

Suppose we have a cascade verification system which is constituted from 2 different verification system. For a test data, maybe just one of the sub systems be used, and for an other maybe both of them are fused and used.

I can compute FMR,FNMR and TMR for each of sub-systems but how can I plot ROC curve for the overall system?the Scores of both subsystems are in different ranges. if there is a fixed threshold for both systems i can do it but if I want to plot the ROC curve I need to do it for different values of threshold, and because my system contains two sub-systems, and these thresholds are different I don't know how to plot the roc curve!!!!!!

an hour ago