See below.
Lordaeron's core can, currently, handle that population. This is without released TBC & WotLK content and areas, which means the server is not required to support as much populated zones & instances as Ragnaros and Deathwing do. Additionally, Lordaeron's core is much newer than the one that Ragnaros and Deathwing are currently running on. So no, currently, it is not going to work. The population of Ragnaros and Deathwing are still at an appropriately acceptable population level, their cores would not support a combined population, and we're not prepared or interested to invest the resources and divert attention to this when it isn't absolutely necessary.