Use of the following collaboration tools is encouraged. You are free as a pair to decide on different tools. However, everyone can benefit from active use of the same tools to learn from each other.
Gitter can be used as a platform to discuss details pertaining to your project. Each pair will have one room for their replication. There is also a community room for the entire team. https://gitter.im/replisims/community
While you could just use github as a mere cloud storage for your code it is most powerful if used in combination with git.
Github issues are an easy way to alert your partner of a particular issue. You can ask for help and assign tasks. This is also an easy way to involve a consultant by tagging them.
Best practice is a branch based workflow. Whenever you have finished a chunk of the replication you create a merge request and ask your partner for a code review. The reviewed commit can then be merged to the master branch.
To ensure the same computational environment for both partners (and potentially an consultant) and avoid problems with R versions, package versions and operating systems it might be useful to run the code in a docker container. Web based alternatives might be binder, code ocean or R studio cloud.