1. Using Canonical tags on all pages of Mobile site to point to corresponding pages of Desktop site 2. Use Alternate tags on all pages of desktop site to point to corresponding pages of Mobile site 3. Make sure Googlebot Destop can crawl both versions of site 4. Googlebot-mobile should only be able to crawl the Mobile site. This can be achieved by adding a 301 redirect from desktop to mobile site (respective pages) when detecting mobile browser/googlebot-mobile
Just follow above steps and you would be perfectly fine with your rankings. We are using this approach and it works beautifully.