I believe in each passenger getting one "space" in the overhead bins. If I check my suitcase I put my backpack overhead. It's much smaller than a roll-aboard and usually has my laptop, some toiletries, and a simple change of clothes or at least underwear in case I get stranded. I've never had any issues with it. I also try to fill in the gaps when placing it in the bins.

More often I choose to carry on to save time at my destination. In this case the roll-aboard goes in the bin with the backpack at my feet unless I'm in the bulkhead seats.