Is it OK to send another reminder after prospective research leader stops responding to my email? Long story short. I was look for some undergraduate level research for this July and August, so I contacted a professor like a month ago. Things went very well, and two weeks ago I even had a Skype conversation with one of their PhDs, from which I can tell that they are preparing to include me into their research group, because we talked about a lot of details like everyone's schedule, and they will presumably be my research leader in the future.

At the end of our remote meeting, the PhD told me they will send me a detailed schedule and research topic to me "in one or two weeks", and "feel free to shoot me an email in case I forgot about this". It has been over two weeks since then, so I sent an (extra polite) reminder three days ago, but I still haven't got any response yet.

Is it OK to sent another reminder to them? How long should I wait before doing so?

This is not a duplicate of How to get people to reply to emails and what to make of a no response?, because I am talking about a reminder to a previous reminder that is already sent. Also, the recipient is a PhD instead of a professor.
You can wait for another 1-2 days and send a reminder again. In the reminder, ask if there is any change in the prospect of the planned collaboration. You can also offer yourself to help with the situation, if they see you fit for this (even if the chances are small that you can actually help).

Of course, keep being polite (but do not over-do it).

Good note from @user2912328 in the comments: verify the correctness of the e-mail address of the PhD guy.

Note from me: please also verify the SPAM folder of your email. A friend of mine found (by mistake / luck) no less than 3 messages from me hidden there - while I was worried that he might be upset with me or something.