YES. In fact you need an assistant by default. Seppuku was done by cutting the stomach sideways, then diagonally upwards. This didn't actually kill the person committing seppuku. In fact it could sometimes take several hours for him to die. The moment the pain set in, the person committing seppuku often fell into hysterics, sobbing and screaming, as he realized that he's guaranteed to die (seppuku is irreversible) slowly, painfully, and helplessly. The assistant stood ready behind him to cut off his head before the onset of hysteria, affording final dignity to the fallen.