The code below illustrates how it can be done by using Metview's built-in field arithmetic:
import metview as mv
# read grib data into a Fieldset object
f = mv.read("my_data.grib")
# add up the values in first two fields point-by-point in their storage order.
# No geographical locations are checked/matched. The fields must have the same
# number of gridpoints.
r = f[0] + f[1]
# an alternative solution
r = mv.sum(f[:2])
# the resulting object is Fieldset with one field only. We can save it to disk as
# a GRIB file
r.write("my_result.grib")
See also the other basic mathematical operators available for GRIB data: https://metview.readthedocs.io/en/latest/api/functions/math.html