It wouldn't make sense -- at least not with only 2. If your primary heir is sick enough to need an organ transplant, they have a good chance of dying. Some organs more than others. But if you need one that classifies as: very sick. If you uh just take one from your only other heir you risk killing them too. People cannot be put together like legos, even with modern medicine.
Heir and a Spare is a tradition going back centuries. Modern medicine has just made it possible for the Spare to be used piecewise.
It wouldn't make sense -- at least not with only 2. If your primary heir is sick enough to need an organ transplant, they have a good chance of dying. Some organs more than others. But if you need one that classifies as: very sick. If you uh just take one from your only other heir you risk killing them too. People cannot be put together like legos, even with modern medicine.