Why are there 2 solution files in my project folder?

there are 2 solutions in my project:

which one am I suppose to be using?

According to the Mastering Unity Project Folder Structure - Version Control Systems article:

testproject.sln and testproject-csharp.sln – solution files for IDEs, first one includes all C#, JavaScript and Boo projects, while the second one – only C# projects and is designed to be opened in Visual Studio, because VS doesn’t know to handle JavaScript and Boo projects.

So what you should be using depends on the IDE.