It sounds like you had some custom code previously that would evenly distribute the menu items between the two columns in your dropdown navigation. This type of functionality is not built-in to Magento out of the box.
There are a couple ways this could be achieved:
1. Custom module with an observer: An observer could help for events like layout_load_before and then do some logic to count the total menu items, divide by 2, and assign the items to the columns accordingly.
2. Javascript: On page load, use JS to count the navigation items, figure out the halves, and move elements between the columns.
The function you require is achievable but would require some custom code. I would suggest your developer look into one of these options. An observer would probably be the cleanest approach. The key is just dividing the total number of items by 2, then assigning the first half to column 1, second half to column 2. Some additional logic may be needed to handle any leftovers if total items is an odd number. But this should give you a good starting point to implement the responsive multi-column nav you want.
