Since we do not know the feature providing the two individual pull requests that are intermingled does not make sense. So you should
1) You should fork upstream project. Then create a branch say "My Cool Feature". This branch is your "origin", and where you forked is "upstream".
2) Then your colleague should fork your "My Cool Feature" branch. Then that is his/her "origin", and his/her "upstream" is your fork.
3) Then you commit your code changes either directly or through another branch into "My Cool feature"
4) You colleague will submit his/her changes in the form of pull request to you, and you will take ownership of that pull and review and merge into "My Cool Feature"
5) Then you update your copy with a "git pull origin (master|branch)" command to get your colleagues changes into your branch
6) Your colleague one you merge does "git pull upstream (master|branch)" to update his/her copy to get the latest.
7) Repeat steps 3-6 until you are done with project, then from the "My Cool Feature" branch submit a pull request to the upstream project for project leads to accept your contribution.
see how to add "upstream" tag to repo here ModeShape Development Workflow typically it is "git remote add upstream URL", origin is implicitly created with the fork.
Hope this helps.