I am making chicken thighs and am planning on using the rendered fat to fry potatoes to go with them. The chicken fat is not quite enough and I have leftover bacon fat. Is there any reason I shouldn't combine them? (I get that the flavors with mix).
It probably won't destroy the universe. I say go for it!
The answer is not so straightforward. Chicken fat has the smoking point of 375F. Lard is 370F. So as long as you are frying below that temperature you should be OK. A lot of french fry recipes recommend double frying first at low and then at higher temperature (often 375). If you are planning on getting the fat to high temperatures, I would recommend using an oil with higher smoking point, and keeping the chicken fat for something else.
I like to fry potatoes in duck fat. so I'd say go for it!