Lets say that P1 is PSD of signal #1, and P2 is PSD of signal #2. The arrays have the same length.
I guess in Matlab it would be:
P1_max = max(P1); %find the max value of P1
P2_max = max(P2); %find the max value of P2
P1_norm = P1/P1_max; % P1 normalized within [0, 1] range
P2_norm = P2/P2_max; % P2 normalized within [0, 1] range
and now you need to plot those two normalized PSD:
figure;
plot(f, P1_norm); %plot normalized PSD for signal #1
hold on; %hold the current figure so it can be used once again to plot another PSD
plot(f, P2_norm); %plot normalized PSD for signal #2 on the same figure
cheers