There is a reason... If I use nemisys players can not check ip of other servers using mods with nemisys they see in mod only ip of main server but with transfer packet they can get ip of my 2nd server and force join without transfering by 1 server
By adding some kind of verification that the player was on server 1 you are able to do that without the need of Nemisys. As dktapps said, it should be dead so it's best to use another way for your needs.
Hormones provides a feature called "transferOnly", which only allows players transferred from other servers in your server network to join a server. It works like players are temporarily whitelisted when transferred from another server in your own network. This feature is not tested though, since I don't have an MCPE client to test