The SolidWorks R&D team is very pleased to announce that the 2019 Beta program will be launched at the end of June 2018.
Their press release
[At SOLIDWORKS, we know that you don't just create drawings, but also that your drawings are built.
To help you streamline and accelerate the entire product development process, from concept to manufactured products, SOLIDWORKS 2019 contains hundreds of new user-focused enhancements.]
You'll note "user-focused" :-) for the others were perhaps not so without us realizing it. In other words, the whole process and not just making parts, drawings and files for the machines.
Question
how to install a Beta version on the same machine without it making a fuss about the 2018 version.
No particular problem to install several versions on the same machine, you just have to be careful to link the right template files (basically ALL the paths in the system option/file locations.
Now to test a BETA version, I would really take a fresh formatted machine, isolated from the general network, to start with a healthy Windows base and be sure not to shoot something important.
If you don't have a physical machine available, maybe look at a virtual machine (WmWare for example) to do tests other than managing large asm, it may be enough.