Pursuing two goals is very common for McKinsey case interviews, especially when they are conflicting to some degree. So always keep that in mind when going into the interviews.
Now for your question, there are several approaches on how to tackle this.
I believe the most elegant way to do this is to create unique top-level buckets for each. Then, once you have presented your structure, you would discuss prioritization and next steps, where you can ''marry'' some of the insights you generated.
Let's say you create 3 top-level buckets that work towards the cost reduction in operations and 2 that discuss a relevant marketing strategy. Now you go into the deeper levels of your structure, presenting concrete ideas that would help the client achieve these goals.
Once you are done with presenting your structure that is
you can go ahead and prioritize where you want to go next. Prioritize items that work in tandem.
For instance, you can say you want to start with measure A for cost reduction and measure D for the marketing plan. State out loud that in combination both measures work to create impact for the client and generate some new insights and implications:
''With our cost reduction in production, we will be able to price our goods lower going forward, I believe that measure A is well suited to achieve this <discuss in more detail why that is>. Now, we can make use of these lower product prices and position ourselves as a lower-cost supplier of this specific good. In turn, we should exploit this with our new marketing strategy to increase awareness of the new positioning and scoop customers from our competitors. Out of my measures discussed, I believe, we should first focus on measure D. <discuss the details>.''