Not sure what the problem is; if you've already got two RPMs, why do you need to make them into 1 rpm? Why not just use a shell script to install them both at the same time?
The RPM command will take multiple arguments, so for example:
rpm -ivh one.rpm two.rpm three.rpm
Will install one, followed by two, followed by three.
Otherwise, from what I know, you're going to have to build the packages first, and then build them into an RPM, at which point you will have a custom RPM that will have to be manually updated every time one of the component packages changes. Yuck.