The rent on a one bedroom vs two bedroom. Food is clearly more for two, especially if baby food is involved or formula. Clearly different groceries for different people. Diapers, clothes, gas and electricity to run twice as many loads of wash, sometimes more in the case of a messy toddler or potty training. Gas to shop different stores, different doctors, school, day care, evening care, more clothes because the clothes he or she was wearing last tuesday no longer fit, more toddler decongestant, more shoes because the shoes he or she wore yesterday don't fit, winter coat in size 2 for this winter, winter coat in size 4 for next winter (hopefully, the woman's coat size will not change), same thing for pants, paint for the walls because child used a marker on the baseboards, money for an ER visit because child stuck an unpopped kernel of corn up his nose and it's now stuck, etc. You get the idea.
How does it NOT double is the question.
(Disclaimer: My kids would like everyone on the DISBoards to know that the events described above are not necessarily my experiences and no inference to that effect should be made, especially with the unpopped corn kernel

)