exactly @Rômulo Alves.
There are 2 problems with sharing the same key across multiple devices:
The unique keys are used to identify you when you are "online" (which we call gossiping). Devices in the network will get confused if they suddenly see 2 devices identifying themselves with the same key at the same time.
There's also the risk of breaking your own index of messages. Each message you create receives the id of the previous item in your database and a time-stamp. If you post a message on device 1 and then post a second message on device 2 (before it has a chance to download message number 1) you'll suddenly end up with a newer message containing an older time-stamp.
I (and most multi-platform users) use 2 separate identities that are "linked" to each other via their bios. I for example have @Hendrik Peter and @Hendrik Peter (Mobile) .