Bindi
You can get a second two-week extension but it usually has to be authorised by a Decision Maker rather than the call centre staff so you must contact the DWP immediately to make the request.
Phone the PIP Helpline first thing tomorrow morning, it opens at 8pm, hopefully, you will hear back in time to void having to send the form in. Give them a call back on Tuesday for an update.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems