You can use the Dashboard! It allows you to create metrics for several different date ranges.
In this case, set up two metrics, one for each email in your split test, and compare the conversion rate - the percentage of contacts who opened that email. Place the metrics side by side in the Dashboard and get frequent updates at a glance. The Dashboard is great for these ad hoc comparisons. This image shows how to create a metric:
You can read more about the Dashboard in our Dashboard Overview article.