The Titanic took 2 hours 45 minutes to sink from the moment of impact. Assuming that the Californian is contacted and responds she's going to take 45 minutes ish to arrive, she was about 12 - 18 miles away iirc so even at a slow ish speed its going to take time.
So assuming you have 2 hours to transfer passengers you'd be using all of the Titanics lifeboats and Californians. Send the boats over from the Titanic, winch them up unload passengers, whilst this is going on, the Californians boats would be over at the Titanic getting winched up and loaded, they come over, Titanics boats go back, repeat, rinse as many times as necessary. The big problem is that the Californian don't have as many lifeboat winches as the Titanic so loading would be slower but you'd still get many off. Sure it would be cramped and uncomfortable up on her decks but you'd have more help on the way. Both ships would probably be sending CQD/SOS's and Carpathia would be on her way to lessen the load.