Yes, it actually is not difficult.
By "user input" I assume you mean that you want to use a prompt for this. You would need to create two formulas that work something like this:
{@FirstGroup}
if {?GroupPrompt} = 'Group1' then
{MyTable.Group1Field}
else
{MyTable.Group2Field}
{@SecondGroup}
if {?GroupPrompt} = 'Group1' then
{MyTable.Group2Field}
else
{MyTable.Group1Field}
You then set up your groups based on these formulas instead of on the fields themselves.
-Dell