It is absolutely okay to have multiple staging sites under a single domain name. For example, if a few people have divergent ideas and you want to test those ideas all at once.
You can create as many staging sites as you like to, all of them available from the same domain like
domain.com/staging2 and so on.
For testing new things this is absolutely ok but it gets complicated if you want feature x from site1 and feature y from site 2.
So if you want to test different things at the same time on different staging sites it makes sense to have one master staging site where you implement all the feature which you want to get on the live site.
So you can merge all the features on one master site and then you can push this staging site to live. The advantage is that you will not run into any conflict.