Yes, you can connect both a tablet and a keyboard piano to this device and listen to them simultaneously on your headphones. It has four mono line inputs that allow you to mix multiple line-level audio sources together into one output, which you can then monitor through headphones connected to the output. Just connect your tablet and keyboard piano to two of the input channels, adjust their levels, and use the output for your headphones. This setup enables you to play and hear both devices at the same time.