Site went down badly yesterday, lol, wasn't able to get on at all.
As for the scenes shown in the end of that video we have to be cautious. We don't yet know when (in the timeline) and where (which realm) this takes place.
In all honesty I never got why people keep insisting that Roxas has to be stuck inside Sora forever. He gave back all the parts that weren't his own (namely the body and soul to Sora and the heart to Ven), yet his own mind, will and memories belong only to him and it was hinted as early as Blank Points that those who called out to Sora can somehow "return" to the world. We see Roxas together with Sora, Donald, Goofy, Riku, Mickey, Terra, Aqua and Ventus, so if I would make an assumption I would say it has to do with something that both Sora and Ventus have to do together. Roxas was created from both of them inadvertently, so maybe it can be done again by both of them willingly without making anyone of them incomplete.
Nonetheless, this is all speculation as the "how is it possible" cannot be answered yet as we don't know how the process "Birth by Sleep" exactly functions right now and how Roxas' "rebirth" is going to happen.
It is a mystery that can't be answered yet just as we could not answer why Ventus looks like Roxas' twin brother before Birth by Sleep was out.
Depending on what happens in DDD we might get the answer there or maybe even only in KH III (As I personally doubt that Roxas will already be back at the end of DDD).