In your discussion of the Germanic zweibach or zwiebach, you reported that they came from the same root. In English we have a complicated rule for selecting ie or ei. "'I' before 'e', except after 'c' or in sounding like 'a' as in neighbor and weigh." Clearly there are very different (and complicated) meanings associated with the pronunciation of "ie". I simplified the words of Germanic origin. I pronounce the word making the second vowel long. So I say to myself, "Zweibach is nice, although zwiebach is sweeter." Am I wrong?

In German, Zwieback is pronounced /tsvi:bak/. In English, zwieback can be pronounced /swi:bæk/, /zwi:bæk/, /swajbæk/, /zwajbæk/. What many call long vowels in English ā ī ō ū are technically diphthongs /ɛj/, /aj/, /oʊ/, /ju:/, but one of them is a vowel ē: /i:/. (The "rule" you stated for English doesn't work for some words: e.g., weird is /wiɚd/ and /wɪɚd/.) German ei is a diphthong and is pronounced /aj/.

Looks like a good mnemonic device to me, seen how we pronounce our neighbour's Zwieback. But even though they bake them zwei times, they never call them Zweiback.
If I'd give my amateur phonetics on the word I would say
/ tsweeback; for zwei /tswhy or tswI.