Some co-authors unresponsive when I asked for consent for submission to biorXiv In short - wrote an article, sent the manuscript to all co-authors and asked for consent for submission and for feedback, if any. After most of the co-authors replied, I made some small tweaks to the manuscript that improved it, but I'm still waiting for consent from 2 co-authors. I sent the e-mail 2 weeks ago, and that should be enough time for them to at least acknowledge they received it, right? All other co-authors replied within a few days.

What makes this especially interesting is the fact that the 2 co-authors that didn't respond were the ones not actually super involved in the experiments, they just provided the transgenic model that I experimented on. I included them as authors on the first (published in a journal) paper because my PI told me we should include them because of that and ok. But now they aren't responding at all, so I'm wondering how to move forward. Remind them? When? Just submit without their consent? Remove them from the author block (and risk a clash with the PI)? I want to submit this, but have no idea what to do.
2 weeks is a fair amount of time, so yes, remind them. You could also write something like "If I don't hear from you by [time] I'll assume you consent to submission", or "I'll submit this by [time], if you have any objections please let me know before then".

That said if you have a PI, you might as well ask them what to do. There's no reason not to, especially for contentious decisions that might lead to bad feelings.