Hi Sam,
as a workaround you can duplicate the installer.jsx/ASSETS folder, and let the first installer.jsx call the second installer after he's done with the first panel, something like:
$.evalFile(File($.fileName).path + "/installer2.jsx");
this would basically chain the two installers (not very elegant I know!)
Also, you may want to have a look at the fine project of Max Penson here: Zxp Installer which basically is his own native, open source version of Adobe Extension Manager (embedding the Adobe ExManCmd commandline utility which has not been discontinued like the GUI version).
For those living in a cave in the last week, Adobe Extension Manager doesn't support anymore CC2015, etc etc (see: HTML Panel Tips #17: CC2015 Survival Guide | Photoshop, etc.)
ZXP Installer works flawlessly, unless you've messed up the ExMan database with the CC app one (I happen to had them both screwed and it's been a majestic PITA for three days).
Shortly, Adobe has killed Extension Manager, transplanting his core into the CC app + the ExManCmd utility, both relying on the same database - Max is using the ExManCmd under the hood of his native application.
Myself, I've spent so many hours doing customer support for Extension Manager, that I personally prefer to use something different now - at least I can blame myself in case something goes wrong :-) but it's just a matter of personal preferences.
Hope this helps!
Davide Barranca
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www.davidebarranca.com
www.cs-extensions.com