Gracehoper welcome to FlyerTalk and the QR forum. Using the dashboard you should be able to find much useful information and hints.
In terms of QRs baggage rules the size restrictions apply to each piece individually in the 'piece' concept whereas the maximum dimensions apply to all your items combined as one in the weight concept.
In your example, using the weight concept, place the two pieces together and you have 75x45x50 = 170. Were the two pieces of different size then the calculation would be the largest width plus the largest width plus the combined depths since it is the size of the combined pieces. You could have up to 1 cbm. but there is almost always waste space due to differing sizes.
You also need to take account of the fact that a single item of luggage must not exceed 32 kg.