Hi Derek,
Sorry I don’t know the answer to you question about running two versions of Spire on the same hardware, however as it is never a good idea to experiment/play/test where you ‘might’ mess up your live installation you could be better off reformatting some old machine with Win10Pro and installing 3.6 on it. You could put the 3.6 client on that same machine and RDP in to it from the comfort of your own desktop.