I think that the only way to do this would  be to merge to the first side,
reverse the order of the data set (or of the cards, then turn them over and
run the merge again so that it prints on the other side

You have to save the values using SET fields and use REF fields to print the
saved values, e.g. suppose you have two fields F1 and F2 to print on the
front and two, B1 and B2 for the back, then you need e.g.

(front)
{ MERGEFIELD F1 }
{ MERGEFIELD F2 }
{ SET R1B1 { MERGEFIELD B1 } }
{ SET R1B2 { MERGEFIELD B2 } }

{ NEXT }
{ MERGEFIELD F1 }
{ MERGEFIELD F2 }
{ SET R2B1 { MERGEFIELD B1 } }
{ SET R2B2 { MERGEFIELD B2 } }
{back)
{ REF R1B1 }
{ REF R1B2 }

{ REF R2B1 }
{ REF R2B2 }

This should work for individual fields. You may have difficulty using this
technique if you are using ADDRESSBLOCK. All the {} are the special field
braces you can insert using ctrl-F9.