こういうこと?
php
1<?php
2
3$data = array(
4    'a' => 1,
5    'b' => 2,
6    'c' => 1,
7);
8$data2 = array(
9    'a' => 1,
10    'b' => 2,
11    'c' => 3,
12);
13$result = array();
14
15foreach ($data as $key => $value) 
16    empty($result[$key]) ? $result[$key] = $value : $result[$key] += $value;
17
18foreach ($data2 as $key => $value)
19    empty($result[$key]) ? $result[$key] = $value : $result[$key] += $value;
20
21print_r($result);
おんなじコードを繰り返すのも芸がないので、
php
1<?php
2
3$data = array(
4    'a' => 1,
5    'b' => 2,
6    'c' => 1,
7);
8$data2 = array(
9    'a' => 1,
10    'b' => 2,
11    'c' => 3,
12);
13$result = array();
14
15foreach (array($data, $data2) as $arr)
16    foreach ($arr as $key => $value) 
17        empty($result[$key]) ? $result[$key] = $value : $result[$key] += $value;
18
19print_r($result);
とか。