I would like to have two separate applications use the same Mongo DB instance, and since I am developing them at the same time I would like to be able to share the same deve
Yeah, you can just start meteor with the MONGO_URL parameter like:
$ MONGO_URL="mongodb://localhost:27017/myapp" meteor
or
$ MONGO_URL="mongodb://localhost:27017/myapp" meteor --port 4000
This assumes you have mongodb installed on your system. See this question for ways to make this process a little easier by using environment variables or a start script.