The main things schools are looking for with research experience are:
1) Have you lead any independent research work and developed the perspective and skills that go with that
2) can you intelligently communicate your research experiences and findings
3) do you have enough experience that you know why you need an MD/PhD to pursue your goals (ie do you know what you're getting into and have good reasons for it).
They are not necessarily looking for a minimum number of years or labs or publications to fulfill those requirements. If you feel confident in your application and convey that well in your essays/interviews then 2 years should be sufficient.