At my workplace, I’ve managed to create a graph of VPN sessions over time using a graph type called device_cras_sessions. Although, with two devices, if you want them summed, you might have to use an external image for your dashboard panel. If you’re not sure how to do that, make a graph for one of your ASA’s with the settings you like, and then right click on the graph and Copy the image location. Use this link, and append the id number of the second ASA to the first device ID number with %2C separating the two.
Example - Call the external image with the following path, starting with graph.php:
graph.php?device=2073%2C&from=1583938079&to=1584024479&width=906.6666&height=355.6666&type=device_cras_sessions&legend=yes&absolute=1