Cakacaka, 33, joins from Gateshead while Ratcliffe, 23, makes the move from York City Knights, and both players met their new team-mates as pre-season training began on Monday night, after signing one-year deals.
Director of Rugby Darren Williams said: “Tabua asked for an opportunity to prove a few people wrong and after a recent trial spell he showed he has the hunger and the desire to succeed, obviously he is a giant of a man and he will put pressure on the other forwards already at the club.
“Danny raised a few eyebrows with his performances during York’s play off campaign and was courted by Keighley and Batley. We already have two quality full backs at the club in George Rayner and Stuart Kain but Danny appealed to us with his versatility and he was willing to back his own ability to force his way into our thoughts. “
Williams added: “We have one further new signing to announce from a Super League club then our squad is complete.”