Concatenate two columns

Need your help again chaps!!

I have a spreadsheet with 2 columns of names and wish to pair them together.

However, the pairing must be the 'lowest cell number' person in column A (usually in cell a2 but not always!) with the 'highest' person in column B eg. B200+. Both columns can be of variable length.

As the columns are not always of the same length, there will be some instances where there cannot be a 'full pairing' and I would like that to be displayed as P.needed / whatever or Whatever / P needed depending on which column is largest.

I've attached a demo if my mumblings are a bit unclear!
I cant seem to find anything on the site which can do this!

Many thanks for looking.