updating a hstore with multiple new columns
you can concatenate them: 

set hstore_col = hstore('new_key_1',foo.new_value_1)||hstore('new_key_2',foo.new_value_2)


Alternatively use the constructor function that accepts two arrays:

set hstore_col = hstore(array['new_key_1', 'new_key_2'],array[foo.new_value_1, foo.new_value_2])


The second solution will be easier to extend when you have more keys then just two

If you don't want to overwrite the current value, then append that to the original one:

set hstore_col = hstore_col || hstore('new_key_1',foo.new_value_1)||hstore('new_key_2',foo.new_value_2)


or

set hstore_col = hstore_col || hstore(array['new_key_1', 'new_key_2'],array[foo.new_value_1, foo.new_value_2])