They will get one draft each. So they will be working on two different unpublished versions. The last published draft will be the current visible version.
The objects are not locked, it's legal to have n drafts of the same object. In 3.1 we've added a warning page where you will see which people is currently editing the current object.
If you get an error message you will not be able to publish and hence the last published will be a valid object.
