Hejsa Henrik,
Thanks for your message.
Up front, please note that we have several tutorials available which might be helpful for you:
Indeed, while the basics seem similar, there some differences making music with the computer compared to do it on iOS.
While Inter-App Audio apps are limited to one instance per app, for apps like SampleTank there are workarounds available, to use these apps in a multi-timbral environment. However this comes with compromises as well.
You might give it a try to get in touch with our friends at IK Multimedia once more.
Best,
Lars