You might want to check the Engl manual. I'm pretty sure the thru is wired in parallel. If you hook up two 8 ohm cabs using the thru port, you need to plug the first cabinet into a 4 ohm jack and not an 8 ohm jack on your amp.
Pretty sure Fantom is right here. It is loaded with 16 ohm speakers, in mono it is @ 8ohms, if you use the "thru right" input jack you need to go into another 8ohm cabinet into 4ohm input on head (but 8ohm input on the head is technically safe as well).
The amp just have one 8 ohm output.
When i see these answords... the two cab's togheter have 4 ohms... so i think it's to dangerous for to blow up the amp.
I Just go for the ENGL cab alone.
It is okay to run at 4 ohms into an 8 ohm load and vice versa, as long as it is not more than double the load (4 into 16 is 4 times and probably not safe for long/at volume) then it is perfectly fine even for the long haul. Companies like Mesa and Hughes & Kettner will put it into writing for you. honestly you have a great 2x12 and will probably gain nothing by using the boss cab, but you can try it and find out. Also you can run the engl in stereo, use just one speaker, (8ohms from the head into 1 16 ohm speaker) and then you have a oversized 1x12 that gives you more tonal and recording options.