I will 100% tell you that she is stressed due to having a cage mate, and low temps. This is a situation that would have been totally avoided given the tiniest bit of research. Instead of saying please help my snake isn't eating. You question was how do I forcefeed. Let me tell you that in my almost 6 years of owning and 4 years of breeding balls Ive never needed to do that. They stop eating for a reason, stress, Illness or cooling. I'm sure there's others that have been doing this much longer than I and have never or had to do it less than a handful of times. What I'm telling you is that force-feeding not only is a last option this snake is going to die tomarrow resort but it's very very stressful and can seriously injure the snake if it's not done right. Not to mention should only be done by a vet or a professional.
I want you to understand that if you separate these two and get your temps right she will be eating within a week or two if everything is correct. This is a simple fix. She's very stressed and too cold. Multiple snake enclosures set up the right way is very advanced and it's actually something I don't even do. Theres just no reason for it. It normally is just set up out of convienence for the owner and in all reality it's turns out to be very inconvenient. Why? Because of issues like this. When theyre not set up right you have one stressed submissive snake and one dominant snake that takes over the whole tank.
So no do not forcefeed. I think you'd be risking way to much by doing it and you're too new to balls to be able to do it the right way. Fix the setup and the snake will calm down and resume eating.