FEATURE REQUEST: Add TotalThroughput RDS metric

I don't know, but you can use CloudWatch Metrics maths expressions to add them up. If one metric is in "m1" and the other in "m2", you can create the expression "e1" defined as m1+m2 to get the total amount that is missing.

In the CloudWatch Metrics console, you can also choose the graph to stack multiple metrics one on top of the other (the "Stacked area" selection on the Options tab), so that you can easily see the total visually, without using metric maths.