The curious thing about double posting on dtube and DLive is, that both use ipfs as backend for storing the data (in the case of DLive not directly, but after the stream is done). Since ipfs is content addressed it will be only "stored" once (meaning that it will create only one unique hash that is used for addressing this content) in the system.

Since ipfs is peer-to-peer, like BitTorrent, the more popular a file is, the more copies of it exist in the network. So if you upload it to both, you have better chances of your video being available for a longer time (even though I can't say how much more, since it depends on how long both services decide to pin the uploaded videos).