Short answer: focus on research, hit normal benchmarks for volunteering, clinical experiences, shadowing, etc. to be safe.
Long answer: On paper, it's said that you should have the same benchmarks for MD applicants (including soft metrics like volunteering, clinical experience, shadowing).
In practice, MD/PhD directors are often slightly lenient with the soft metrics, in exchange for significantly more emphasis on your research - not only on the quantity of research hours, but the quality of your research experience. They will be looking to assess how well you can explain your project and field questions.
There are some administrative considerations behind this that widely differs between programs (i.e., whether a MD/PhD office is able to accept applicants directly without consideration of the MD admissions office; vs. both offices have to accept the student, but the MD/PhD office accepts first and recommends to the MD office who usually agrees; vs. the MD office has to accept the student first, and then the MD/PhD office can accept, etc.) In general (though not 100% of the time), MD/PhD directors that can operate more autonomously place significantly more emphasis on research. This is why advice tends to be quite different and subjective - don't get lost in the weeds.
