Some pairings may go together. For example, pediatrics and ultrasound is a great new field with only a few members. Simulation and almost anything else would be a great pairing that is complimentary. International medicine and almost anything is a useful pairing. So, try and imagine what your career goals are and pick the training that makes sense.
Here are some other pairings that off the top of my head make sense:
critical care - ultrasound
critical care - simulation
critical care - tox (previously mentioned)
pediatrics - critical care
pediatrics - ultrasound
pediatrics - simulation
critical care - simulation
ultrasound - simulation
international medicine - simulation
international medicine - pediatrics
international medicine - wilderness medicine
international medicine - maybe critical care?
research - anything
international medicine - health policy
pediatrics - health policy
critical care - health policy
ultrasound - health policy
ems - health policy
ems - simulation
ems - pediatrics
ems - wilderness medicine
pediatrics - tox
critical care - tox (previously mentioned)
ems - tox
tox - health policy
tox - wilderness medicine maybe?
Im sure there are others, but these are just off the top of my head...
Now to the other people's comments, my experience suggests:
1. People with fellowships are often paid more per clinical hour. This is because there is often protected time involved in advancing the subspecialty within emergency medicine which makes the hourly clinical wage higher but the overall number of hours worked may actually be greater (lots of time put into ultrasound for example). So dont do it for the money as you will generally work harder per dollar. There is lost income associated with the fellowship itself as outlined but there are dividends paid on that investment everday thereafter.
2. The idea that doing a fellowship is only to spark a research niche, I would disagree. I would recommend a fellowship if you are passionate about the subspecialty, and in general wish to become a leader on the ward, in the legal forums, in organized medicine, in education, and research, of that subspecialty. So do things that you wish to continue to immerse yourself into for the rest of your life.