Killing two bears in one minute seems like it would be a difficult thing to achieve. Fortunately it is anything but! You see the good thing about bears (for the challenges anyway) is that they are aggressive and will attack you on sight. Pressing 6 successful button prompts will have Shay kill them. We can use this to our advantage.
Once you have accepted the challenge, head north and climb the ladders here to find a pit below with a pair of bears in it. From the top of the ladder, use the treetop path here to get above one of the bears and hit it with an air assassination. Quickly skin it and then run up to the second to have it attack you. Complete the button presses to kill it.
Once you have the required pelts, the challenge will be complete.