It may not be a pure SW issue. Depending on how the HW is designed, there may be limitations in HW that keeps both tuners from recording at the same time.
For example (I've ran into this very issue in the past), depending on what SOC (System On a Chip) CPU is used, there might only be one DMA channel (Direct Memory Access engine) to move data from the sat decoder to the disk drive. Hence, even though there are two tuners, only one tuner can stream data to the disk drive.
Little design compromises like this in the initial design planning, meant to reduce parts costs, can have unforeseen consequences later on in the ultimate operation of a realtime device.
