I’d class it as two master cylinders, but only one reservoir. The sram lever, when pulled, works as normal, with all the fluid displacement going to the caliper as the cross top lever is fully retracted with nowhere else to go.
When the cross top lever is pulled, a seal on that master piston passes over and closes the port to the sram lever and then acts on the slave pistons at the caliper.
Release the cross top lever and the master piston fully retracts and exposes the port to the sram lever again and more importantly, the reservoir, which can then compensate for any pad wear or fluid expansion that just occurred.
If the cross top lever didn’t close off the route to the sram lever when pulled, it would just refill the reservoir before moving the caliper pistons.