Voxel - There is no limit to the number of programs you can rank. In your case, you would rank the radiology residencies first and each residency would have another rank list for your internship year. For example:
1. MGH Radiology
a. Boston U - Medicine Prelim
b. Baystate - Transitional Year
c. St. Vincent's - Transitional Year
d. Mt. Sinai (Cabrini) - Medicine Prelim
2. Albert Einstein Radiology
a. St. Vincent's - Transitional Year
b. Mt. Sinai (Cabrini) - Medicine Prelim
3. St. Vincent's - Transitional Year
4. Baystate - Transitional Year
The main match (numbered programs) will determine which internship rank list (lettered programs)gets used (if any). Some people will rank some TYs or prelims (I wouldn't recommend ranking medicine prelims separately) just in case they don't match for residency. I hope this helps.