It all starts off nice and calmly. I ask the DC (5 and 7) if they can do something in a normal speaking voice. They carry on talking loudly to each other, in whatever game they are playing, and I am not heard. I ask them to stop talking while I talk. It doesn't happen. Repeat during the day 100 times. They end up doing exactly what I have asked them not to do, and not what I have asked them to do. = SHOUT. I am not shouty, but the repetitiveness of this ordeal makes me just want to cry sometimes!
The second issue is that there is no threat or punishment that I can carry out that actually means anything to them, or that they care about.
I'm in the same boat here. Two kids 3 years apart, both never hear me unless I call them for an ice cream.
I have to really concentrate and remove whatever they're doing. Once I'm in line with their eyes I get a bit of attention. It's a start! I have to be careful not to overdo it though, or I will end up being invasive and that wouldn't be fair on them either.
Usually I find if I target the older one, the other will follow, but again, I don't want the eldest to grow up always being the one targeted for Mum's instruction! It's a bit of a minefield. Certainly it requires more close-quarters interactions, which isn't always practical or convenient. You have my sympathy! I hate shouting but I've turned into a shouty Mum.