How should I approach my advisors about switching research topics, when they're counting on my results for a grant proposal?
This is a really complex problem, especially since you're in your 2nd year of PhD, meaning it left you more than 1 year, right ?

Why are you not getting along with one of the advisor ? Is it not one of the main issue, which gives you the feeling you don't like your research area ?

Breaking plans of your advisors is really not a good idea, there is always gap in motivation when you are in PhD, and I think you should think about what gives you the feeling you want to go somewhere else. That will hurt both your carrier and your relationship with everyone in the lab.

There is no good answer to your question, either way, you will end up with a problem (for yourself and for your advisors). The best way is to speak with them about your feelings, after having a self-reflexion.

EDIT: Since it is clear you don't want to stay in this lab, the earlier the better. Tell them about what are in your mind, and they will certainly understand, if they are not short-minded (which hopefully is not the case for a scientist !)