I'd suggest browsing the
Combined Residencies forum here, although don't post those questions since they've been addressed many times before.
You can also find lists of approved residencies leading to double or triple board eligibility on FREIDA (if you don't know what that is, Google is your friend).
Common combined residencies that I can think of off the top of my head:
- Internal Medicine/Pediatrics (4 yrs)
- Internal Medicine/Psychiatry (5 yrs)
- Internal Medicine/Emergency Medicine (5 yrs)
- Psychiatry/Pediatrics/Child and Adolescent Psychiatry (not sure of length)
Of these IM/Peds is by far the most common. There are also some combos involving neuro and family medicine, I think.
Why would someone do them? Often because of interest in both fields and a desire to have a career that in some way combines them. It is much debated, just as with dual degree programs, whether this is realistic and whether most people just end up practicing in one field anyway.