Yes you can bring an under-seat backpack and your carry-on. I am not sure about the restrictions on backpack size, but as long as it can fit under the seat expect no issues. Both need to be under 7kg. The 7kg limit does not include laptops, so you can remove your laptop if they ask to weigh your bags.

In practice, weighing carry-ons IME is extremely rare, but does happen at the gate from time to time. When they do this, usually they pick out the bags that 'look' heavy, so as long as your bags are a small size, the weight limit isn't much of a concern in practice.