Using Workbench Command

MERGE OR SPLIT ON SERIES, SCALAR, OR LABEL DIMENSIONS
   wb_command -cifti-merge
      <cifti-out> - output - output cifti file

      [-direction] - merge in a direction other than along rows
         <direction> - the dimension to split/concatenate along, default ROW

      [-mem-limit] - restrict memory used for file reading efficiency
         <limit-GB> - memory limit in gigabytes

      [-cifti] - repeatable - specify an input cifti file
         <cifti-in> - a cifti file to use data from

         [-index] - repeatable - select a single index to use
            <index> - the index number (starting from 1), or name

            [-up-to] - use an inclusive range of indices
               <last-index> - the number or name of the last index to include

               [-reverse] - use the range in reverse order

      Given input CIFTI files for which mappings along the selected direction
      are the same type, all either series, scalars, or labels, and the other
      dimensions are index-compatible, this command concatenates the data in
      the specified indices/ranges along the selected direction (by default, on
      typical 2D cifti, concatenate horizontally, so rows become longer).  The
      direction can be either an integer starting from 1, or the strings 'ROW'
      or 'COLUMN'.

      Example: wb_command -cifti-merge out.dtseries.nii -cifti
      first.dtseries.nii -index 1 -cifti second.dtseries.nii

      This example would take the first column from first.dtseries.nii,
      followed by all columns from second.dtseries.nii, and write these columns
      to out.dtseries.nii.