Are you looking for an automated way or just a (manual) tutorial on how to do it at all?
I would probably open both scripts (let's call them A and B) in separate Aegisub instances. Open the styles managers for each. Then in B's styles manager rename all styles that have a name that is also used in A (make Aegisub change all instances of the style in B). Then in A's styles manager "Import from Script" B. Then leave Styles Managers from both. For B press ctrl+a followed by ctrl+c (mark all lines, then copy to clipboard). Then in A do ctrl+v (paste from clipboard). Save A and that should be it, I think.
I haven't tested it but the seems to be a python script to do this automatically: https://github.com/rysmarie/Aegisub_merger