Sorry, here are examples that come up for me all the time.
1--entering new demographic information for each of 4 siblings; At the moment, I print out one and data-enter from the paper.
2--being asked,'What was that medicine you put her brother on? It worked great.' Now I have to forward the current chart, pull up the other chart, then pull up the first chart. If it were paper, I could open the second, get the needed info, then close the second and put it away without ever closing the first chart.
3--examining twins. How do I compare one growth chart to another except on paper or printed to pdf?
4--comparing lab results for siblings. If I do the comparison for hemoglobin and the parent then asks, 'OK, but are the white counts similar?' Again quite tediously I have to close one and open another, then close that one to get back to the patient being examined.

For me, it is an ongoing pain and disadvantage to be able to open only one chart at a time.