Combining complex ‘out of phase’ signals does not necessarily lead to complete cancellation. In fact, in the real world, it rarely, if ever, does so.
Sawtooth wave has every harmonic present, so, if the fundamental (the first harmonic) lies at 100Hz, the second harmonic will be at 200Hz, the third at 300Hz… and so on. Adding two of these sawtooths (sawteeth?) with the fundamentals offset half a cycle means, of course, that the fundamentals are cancelled out. But the second harmonics, lying at 200Hz, will be added! The third harmonic, at three times the frequency of the fundamental, will be cancelled out, the fourth harmonic will be reinforced, the fifth cancelled… and so on. The result is a waveform with harmonics at 200Hz, 400Hz, 600Hz and so on… in fact, it’s a sawtooth of the same amplitude but exactly twice the frequency of the originals.