Combine two MT4 server databases together, its users, orders databases with corresponding group and symbol settings.
All you need is to pass two MT4 installation folder path as command line argument and program will prepare result server with ready to go settings.
When you run new server you will notice all users and their orders exist in right place.
Also, it take care about symbol settings which can be different.
After all, clients will be able to connect to new server and continue trading like nothing happened.
Comprehensive documentation can be found here: http://confluence.cplugin.com/display/TOOLS/Join+two+MT4+Servers