class ActiveFedora::Base

m.update_attributes({"fubar"=>{"-1"=>"mork", "0"=>"york", "1"=>"mangle"}}, :datastreams=>["my_ds", "my_other_ds"])
or
m.update_attributes({"fubar"=>{"-1"=>"mork", "0"=>"york", "1"=>"mangle"}}, :datastreams=>"my_ds")
use the :datastreams argument like so:
If you want to specify which datastream(s) to update,

As in update_attributes, this overwrites _all_ available fields by default.

will be overwritten.
An index of -1 will insert a new value. any existing value at the relevant index

This will result in any datastream field of name :name having the value [a,b]

{{:name=>{"0"=>"a","1"=>"b"}}
must look like this :
A convenience method for updating indexed attributes. The passed in hash

def update_indexed_attributes(params={}, opts={})
  if ds = opts[:datastreams]
    ds_array = []
    ds = [ds] unless ds.respond_to? :each
    ds.each do |dsname|
      ds_array << datastreams[dsname]
    end
  else
    ds_array = metadata_streams
  end
  result = {}
  ds_array.each do |d|
    result[d.dsid] = d.update_indexed_attributes(params,opts)
  end
  return result
end