Please specify the exact device you're trying to plug into.
I've never seen or heard of a dual 2.5mm audio configuration (and I've seen and heard of quite a lot of things over my lifetime). I doubt you'll find such an adapter commercially. You might have to make one or get a headset made for the device.
It would be clunky, but if I understand properly you should be able to use the 3.5-to-2.5 adapters that Laptop_Nerd suggested above combined with a 3.5mm headset/mic splitter cable like this: