If I install 6 can I still use 5?

[wim (Wim Dekeyser)]

So I guess that since June last year things changed for the worse then?

Ref. last sentence [as in “final words at that time”] (by @brian) in this thread:

Using Rhino BETA on Multiple Computers

Hi @amanda, your workflow is exactly what we have in mind for licensing using your Rhino account. I encourage you to try it now and see if you can get it to work in all the scenarios you imagine. You can turn off your network connections to pretend you are on an airplane, and see how it works. I’m disappointed by your choice of metaphor. That aside, there are already people linking multiple instances of Rhino/Grasshopper together to distribute workload to multiple machines. Flux and MetaHoppe…

And also:

Rhino 6 License Validation Changes

No. A user is welcome to have R1 - 6 on the same machine… assuming they purchased Rhino 1.0 and then upgraded a number of times. What they can’t do, is run the R1 and the upgrades on many different machines.