you just click File>Open and select as many unity files you want, then in the dialog press Open and wait for load all.
then you can sort by type in the GUI and select the first and hold shift and select the last to select all of that type
or select only what you want with ctrl + click and use the plugins to export to usable files.

edit
if you are working with assetbundle files that only open 1 at a time you can batch extract all those to assets files
that you will be able to batch open with UABE afterward. you need 2 files for this: (well i'll just post what i use

)
1. batch.txt file with this in it
Code:
+DIR C:\temp
2. UABE.bat file with this in it
Code:
AssetBundleExtractor.exe batchexport batch.txt
i have those 2 files next to the AssetBundleExtractor.exe and i use a folder named "temp" located in C:\ to drop
the assetbundle files into, then i just run the UABE.bat file to export the .assets files from them to that same
temp folder,and from there you can batch open those in UABE.

Hello, I used this batch method and successfully opened the file in batch. However, when exporting PNG images, you will be prompted [unable to open the streamed data file! Unable to convert the texture data!]