Is hemodialysis the only answer to a failed kidney if the other one is also weak?

No. Kidney transplantation and peritoneal (home) dialysis are other options when the total function from both kidneys added together is too low to support life and health.
No. Renal transplantation is the best treatment option whenever your kidneys fail. However if you do not qualify for a kidney or are waiting for a suitable donor, you have to perform some form of dialysis. If you younger, independent and motivated, then you can use peritoneal dialysis or home hemodialysis as a treatment option instead of in center hemodialysis.