allow for two and/or three files

data_override/README.MD

@@ -24,12 +24,12 @@ If it is desired to interpolate the data to a region of the model grid. The foll
 - **lat_start:** The starting longitude in the same units as the grid data in the file
 - **lon_end:** The ending longitude in the same units as the grid data in the file
 
-If it is desired to use multiple(3) input netcf files instead of 1. The following **optional** keys are available.
+If it is desired to use multiple(3) input netcdf files instead of 1. The following **optional** keys are available.
 - **is_multi_file:** Set to `True` is using the multi-file feature
 - **prev_file_name:** The name of the first file in the set
 - **next_file_name:** The name of the third file in the set
 
-Note that **file_name** must be the second file in the set. **prev_file_name** and **next_file_name** are required if **is_multi_file** is set to `True`
+Note that **file_name** must be the second file in the set. **prev_file_name** and/or **next_file_name** are required if **is_multi_file** is set to `True`
 
 #### 2. How to use it?
 In order to use the yaml data format, [libyaml](https://github.com/yaml/libyaml) needs to be installed and linked with FMS. Additionally, FMS must be compiled with -Duse_yaml macro. If using autotools, you can add `--with-yaml`, which will add the macro for you and check that libyaml is linked correctly.

data_override/include/data_override.inc

@@ -587,7 +587,7 @@ subroutine read_table_yaml(data_table)
             & is_optional=.true.)
           call get_value_from_key(file_id, entry_id(i), "next_file_name", data_table(i)%next_file_name, &
             & is_optional=.true.)
-          if (trim(data_table(i)%prev_file_name) .eq. "" .or. trim(data_table(i)%next_file_name) .eq. "") &
+          if (trim(data_table(i)%prev_file_name) .eq. "" .and. trim(data_table(i)%next_file_name) .eq. "") &
             call mpp_error(FATAL, "The prev_file_name and next_file_name must be present if is_multi_file. "//&
               "Check your data_table.yaml entry for field:"//trim(data_table(i)%gridname)//":"//&
               trim(data_table(i)%fieldname_code))