I would create zones for each individual field like you are doing and then merge at the end.  To do this, turn on only the maps you wish to merge together and use the Merge All Visible – Equalized script to merge them into one field.  I would also consider leaving them as two separate fields and merging the Rx maps (using the Merge All Visible script) if they are wanting to spread them as one field.  Once the two fields have been farmed as one for a few years then making one zone map would likely work out okay.