Liferay 7.0 CE was the version where clustering was not built in, you'll have to compile/install an additional module to make it cluster aware. It's available and you can find documentation if you google for it - my recommendation though is to invest the time rather in upgrade than in compiling this extra module: We're now at 7.3, you can't really expect any bugfix and will fall back more and more.

Also note that clustering involves more than just pointing multiple servers to the same database and file system. It's well documented what you'll need to do - those are approximately 7 steps and I'll recommend to go strictly by the documentation.

What you describe ("can't see the documents") most likely is a cache issue - e.g. when you restart, and/or reindex, they probably appear. This is a symptom of missing steps in clustering setup. And to go through all of the steps you'll need to first make your server cluster-aware, either by compiling and deploying the cluster module, or by upgrading.

The path of least resistance is the upgrade.