With Drupal Views, how can I hide a Field if another Field is not empty? In other words, how can I show a Field unless its empty, then show a different Field?

I have the following;

Node1
fieldA - text for field A in Node 1
fieldB - text for field B in Node 1

Node2
fieldA - (empty)
fieldB - text for field B in Node 2

I want to show fieldA, unless it's empty in which case i wanted to show fieldB. So my View would show;

'text for field A in Node 1'
'text for field B in Node 2'

How can I do this? I can hide a filed if its empty, but how can I hide a field if another field is not being hidden?

In case it matters, the reason im doing this is im using image cache to generate thumbnails for a gallery. However for some thumbnails I want to be able to upload a different image to be used as a thumbnail.
Thanks