It can be done.
You must take a few steps to set it up, but then you are able to count 7 minutes. The setup makes you be able to time 2 minutes on the 5-minute sandglass and then you do another 5 minutes on the same sandglass.
Setup:
Start both sandglasses at the same time. When the 5-minute sandglass runs out, the 8 minute sandglass has 3 minutes of sand left. At this moment, start the 5 minute sandglass again.
When the 8-minute sandglass finishes the remaining 3 minutes, the 5-minute sandglass has 2 minutes left.
Actual timing of 7 minutes:
Let the 2 minutes pass, and immediately turn over the 5-minute sandglass to count an addition 5 minutes. The last 2 minutes of the 5-minute sandglass plus an additional 5 minutes of the 5-minute sandglass add to 7 minutes.
