Modding split is not hard as you think. Only difference is you need to sign them all and install with Split APKs Installer app
Before you start modding split apk:
Check if APK still support 4.4 and below which provides single APK using Apkcombo Force download ARMv7 APK on APKCombo - platinmods.com - Android MODs | iOS MODs | Tutorials & More!
If apk is not outdated and still provide single APK, mod single APK instead to save your time a little bit
Important: Always focus on ARMv7 libs and optionaly ARM64. ARMv7 libs works on ARM64, x86, x86_64 while ARM64 libs works on x86_64. Don't waste time mod x86 libs if ARM libs work on x86 devices/emulators
Backup split APK from your ARMv7 device: How to backup/export split APK (App bundles) (NO root & root) - platinmods.com - Android MODs | iOS MODs | Tutorials & More!
Or download split APK from Apkcombo. APK Downloader - Download APK from Google Play Store™ (Online)
Select Architecture to make split APK links appear under Download
If you have .apks file extension which contains split apks, extract them all using Winrar or 7-zip
Mod it as you normally do, Inject toast in base.apk using Auto Toaster and replace/add lib in split_config.(Architecture).apk whatever...
To keep it unsigned, simply don't sign and zip them all into .zip file using Winrar or 7-zip
To sign apks, you must sign ALL split apks no matter if it's modded or not. All must be signed because all apk must have same signature. Split apk with different signature and another split apk with different signature is a no no. You will get an error that signature is invalid
You can use APK Easy tool to drop apks on "Sign APK" button to mass signing
Zip all signed apks into .zip file. Filename doesn't matter as long as correct split APKs are included
Rename to .apks if you want, SAI app supports both .zip and .apks file extension
Do not forget to tell peoples to use Split APK Installer (SAI) app to install split APKS file: How to install/sideload split APKs/zipped .APKS file (NO root) - platinmods.com - Android MODs | iOS MODs | Tutorials & More!