It would depend on the state. Podiatric physicians in some states treat soft tissue up to the thigh, some the knee, etc. Some state laws don't really specify. I'm not sure I'd want to manage another doctor's amputation anyway!
Jonwill is correct in his posting that it really depends on the scope of practice in the state. In fact, there are some states that do not permit a DPM to do amputations beyond TMA.