If it is a DIY carbonator cap made with a tire valve stem remove the valve stem core from the fermenting bottle otherwise I don't think the CO2 will exit that bottle. Additionally, if you want to remove the possibility of blowoff getting into the soda add an additional bottle between to catch the blowoff. Also, near the beginning of the process you may need to purge air out of the soda bottle (and the middle bottle if you go that route) otherwise as the fermenting bottle creates CO2 the lighter air in the top of the bottle will be what goes to the soda bottle first.
Good luck