Merge parallel groupings

#1

Hello,

Say I have the following query:

> SELECT COUNT("water_level") FROM "h2o_feet" WHERE time >= '2015-08-18T00:00:00Z' AND time <= '2015-08-18T00:30:00Z' GROUP BY time(12m),"location"

name: h2o_feet
tags: location=coyote_creek
time                   count
----                   -----
2015-08-18T00:00:00Z   2
2015-08-18T00:12:00Z   2
2015-08-18T00:24:00Z   2

name: h2o_feet
tags: location=santa_monica
time                   count
----                   -----
2015-08-18T00:00:00Z   2
2015-08-18T00:12:00Z   2
2015-08-18T00:24:00Z   2

How do I sum the results for each of the time intervals across different groupings? I want to see something like this.

name: h2o_feet
tags: location=santa_monica+coyote_creek
time                   count
----                   -----
2015-08-18T00:00:00Z   4
2015-08-18T00:12:00Z   4
2015-08-18T00:24:00Z   4