Append Collections together

I have two collections, both have a structure like this:

id    trips_in
1     5
2     10

id    trips_out
1     6
2     8

My question is how can I combine them into a single collection like such:

id    trips_in    trips_out
1     5           6
2     10          8

I found out about mapReduce, but its functionality looks like more than what I need.

via ffritz