This is a basic techinique, by John Mark Osborne, on the DatabasePros web site, Resources. You can go there and search for example files. Many of them are oldish, but well done. I'll put a direct link to where the file is here:
It is basically a portal whose relationship is only valid according to the conditions you set. If it matches, whatever is in the portal shows; it could be field(s), buttons, graphics, whatever (except another portal, though there are exceptions). If it does not match, there's nothing there; buttons will not work (though it will still show the "hand" on hover, it's not that smart).
What it cannot do is make two enterable fields share the same space on the layout (not in a way that is useful). So you'd need to offset them somehow.
And if using vs. 11, you can use two one-row portals of a self-join table occurrence (make it invisible) and attach your criteria directly to the portal which would return Boolean true result.
First portal would hold field1 and portal filter would be: Field A = 1
Second portal would hold field2 and portal filter would be: Field A = 2
And as Fenton explains, the fields (with their portals behind) cannot share exact same space. :^)
Thanks to both of you, I' pretty much resolved the task with another approach when the answers came through, but I've made a note of both processes because they can be used a little further down the line in the database I'm working on.