Please cite the relevant passage in the Constitution.
The Constitution only allows Congress to intervene in the case of a tie. They can't "decide neither is fit for office", because the only two requirements are age and citizenship, and Obama threw the latter out.
It has to do ultimately with the examination of the electoral votes, and ultimately immediate impeachment of the winner especially if the loser was so close in popular votes the population starts to come part at the seams!
If in case of an electoral tie that’s where the fun would start! I expect all sorts of shenanigans anyway..such as a stae going for Trump in the popular but some electors go for who ever they want.
Remember both candidates are very deeply flawed and a case for impeachment can and probably will be made for both of them!