Hi @Earendur,
Perhaps someone with more relevant experience in the specific field can give you a more concrete answer.
The way i think of thinks like this when i come across such situations is first i need to estimate the scale of the situation. How many cases have two mutations? are there any of three or more? are there many different combinations? or perhaps a specific one is typical for pairs?
You can use the text explorer on the mutation column to see the distribution of all individual and combinations.
Once done with that, you need to make sure to define the unit of analysis correctly and whether the observations are independent, repeated or nested?
Any answer you may have to any of the questions my help you establish the correct data structure for your analysis.
Will you be using the mutation as an independent or dependent variable? Could the unit of analysis be the mutation? or has to be the patient? if it is the patient, the second mutation may be another row in the data that may require adjusting for non independent observations. if there are too many mutation categories, perhaps you want to aggregate them in a meaningful way and that will also include the combinations.
hope this helps at least a little,
ron