www.makemkv.com

I do the majority of my ripping on an iMac but sometimes use my home theatre PC which often manages discs that the Mac fails on.

Do you have any issue with me using a key on both devices or should I be buying two?
Thanks.

Mike will give the definitive answer, but...

The MakeMKV key can be used on multiple devices. The prohibition is on using it on multiple devices at the same time, so if you sometimes use it on different machines, you should avoid using it on all of them at the same time.

But, unlike some other software, MakeMKV doesn't "phone home" to check if it's in use right now, so...