I have now got Watusi working on my iPad in companion mode. Here is how to do it:
1. Use a signing service or something like Altstore to sideload apps on your iPad. There is no need to do this on your iPhone.
2. Download the Watusi IPA from
here.
3. Install the IPA onto your iPad.
4. Follow the instructions in the app to link to your iPhone.
There are a couple of limitations. Firstly, it is only possible to use WhatsApp on iPad in iPhone mode either zoomed out or in. There is no universal app. Secondly, notifications don't work. Thirdly, iCloud backup isn't available, but that will happen from the primary phone.
It's not a perfect solution, but it works well and is a native WhatsApp experience.