With convertX2DVD you can do a batch process so that they will convert in series (one after the other) rather than in parallel. I have done it and this is what the batch looks like:
"C:\program files\vso\ConvertXtoDVD\ConvertXtoDvd.exe" "C:\program files\vso\ConvertXProjects\Your Project DVD 1.xToDVD" /auto=true /close
"C:\program files\VSO\ConvertXtoDVD\ConvertXtoDvd.exe" "C:\program files\vso\convertxprojects\Your Project DVD 2.xToDVD" /auto=true /close
"C:\program files\vso\ConvertXtoDVD\ConvertXtoDvd.exe" "C:\program files\vso\ConvertXProjects\Your Project DVD 3.xToDVD" /auto=true /close
"C:\program files\VSO\ConvertXtoDVD\ConvertXtoDvd.exe" "C:\program files\vso\convertxprojects\Your Project DVD 4.xToDVD" /auto=true /close
With this method you need to create your DVD projects in Convertx2DVD first and save them. Save this text as a batch file and you're ready to go. Just double click on the batch file and the process will take over from there.
I had never done this, I wouldn't know how exactly or if it works. First you need to obtain and learn to use Convertx2DVD. Save each project and then write a script, then execute.