Just use a second (foodsafe) pail and split the batch in half. Split your chemicals and additions (sugar etc) between the two. It will all end up in one single "secondary fermenter" anyway. (I'm assuming")
If you are feeling flush with money, you can go buy a larger primary fermenter. Places like Sam's Club (I think) and others have these ... maybe even Home Depot. You would want to look for a plastic "garbage can" that specifically has the "NSF" logo some where on it. That is the indication that it is considered food-safe.
The ones I use are Rubbermaid "Brutes" which are 30 gallon NSF rated "garbage cans". They are about $30 each but you can usually also find pails in the 10 gallon and 20 gallon range as well. Like I say ... just make sure the "NSF" logo is somewhere on the pail (usually on the bottom).
The logo will look something like this ...
btw: pails that originally came with food in them are considered foodsafe ... so ones that you might pick up from your local friendly Burger King etc (or Jimmy Johns for their 6 gallon pails ... woot!) are good to use.