ｘ と ｙ に行ったことがない。
ｘ にも ｙ にも行ったことがない。
When I came up with these sentences and asked the difference, my language exchange partner was stumped, but insisted there is a significant difference. Perhaps the も example implies that x & y are only two of many places which haven’t been visited yet while xとy does not?
I pointed out that we can use も in situations like the following, although perhaps this is more simple than the above?