df1 :

id            positive_action    date             volume
id_1          user 1                  2016-12-12       19720.735
              user 2                  2016-12-12       14740.800

df2 :
id            negative_action        date             volume
id_1          user 1                  2016-12-12       10.000
              user 3                  2016-12-12       10.000

I want :

id            action        date             volume
id_1          user 1         2016-12-12       19730.735
              user 2         2016-12-12       14740.800
              user 3         2016-12-12       10.000