Have you tried talking to her about how you're more doing the first one for her and how the second one is more your cup of tea? I've found if writers do make two threads it's usually because both people favor them differently and if that's the case, it's better to be honest about it so that both interests are being served fairly. She should know you like the second one and that it's unfair of her to write less one something she agreed to do. 

Being upset is understandable and favoritism is petty. It's tiring to be in a cycle of excrement and then annoyed and then upset, she should know that what she's doing is making you uncomfortable and that you're not obligated to keep up with the first thread, but you're doing it out of politeness and friendship and that she should be doing the same, for the second one, if nothing else. If you wanna keep both going, you should do so under the agreement that you both favor the opposite but should strive to give both the same attention/effort or it's an arrangement that's only gonna get worse and continue to hurt feelings/expectations and I don't want that happening to you. :/