The test had to start somewhere. There are a number of reasons why they couldn't simply do a 6 person stay, that make it somewhat difficult at least. These include:
- There is a natural cycle to bring down astronauts from the ISS, in that a Soyuz needs to be replaced every 6 months. While in theory a crew could bring a Soyuz, and bring the old one down, that hasn't been done in some time.
- NASA is required to follow OSHA standards, although they have a partial exemption for one particular aspect, namely that of radiation. My answer to this question gives the requirements, which are different for men and women. Women are more sensitive to radiation, because breast cancer is a bigger risk in women, and all other cancers are roughly equivalent between the two sexes. This means that the ideal candidate from NASA for a long duration test is an older male (But not too old).
- There are a lot of astronauts who want to go up to space, but very few who currently have that opportunity. Reducing the opportunity to get to space makes astronauts unhappy.
- Certain experiments that are running on the ISS require special training. There are more than just a long duration space flight test required. Thus, a new flow of astronauts is required.
- There is one NASA and one Russian doing this test. That seems required to balance the ISS workload.
- NASA chose a person with an identical twin, to give more exact comparisons.
- No doubt there will be extra tests done on the two astronauts, which would take away from required astronaut duties, if not handled in limited doses.
Bottom line, they could have done this as you proposed, but they want to start slowly. This will give them valuable data, without causing risks. I'm sure if they learn anything interesting from this, they will continue doing similar tests.