d = {'learning': {'DOC1': 0.14054651081081646,
              'DOC2': 0,
              'DOC3': 0.4684883693693881},
 'life':     {'DOC1': 0.14054651081081646, 
              'DOC2': 0.20078072972973776, 
              'DOC3': 0}
}

d['sum'] = [d['learning'][k] + d['life'][k]
            for k in d['learning']]
print(d)