The issue i can see is that no single saved search is going to be able to combine the two unrelated records into the same line. The one-to-many (parent-child) relationship from Tasks to a Project is a problem for saved searches since you can't do arbitrary join conditions, you can only retrieve linked records.
One way you might resolve that: you could create two new custom Project fields, called Design Task and Install Task, and in whatever code is creating the tasks, add code to update the relevant project field with the task internal id. Or, if they are created manually, create a User Event to populate those fields on the Project. Then, you will be able to combine values from both record into the same line in a saved search.
Another possibility: the new Analytics Workbooks might actually work for this without additional custom fields, though I have only played with them briefly, and have no idea if you can use them for a scheduled workflow. If so, you might be able to define the joins to get fields from both tasks at the same time. I have done something like that with the
N/query
script module once before, but can't find the code I used.