Hi, wonderful idea! :) I feel like that all the time and have seriously suspected my mom to keep the sad truth of my still-born twin from me all of my life. :)
Anyway, I just signed up because I saw this blog post linked to on facebook, and I’m German, and Doppelschmerzen does not translate to “twin ache”. I am sorry.
The word for “twin” is Zwilling, pronounced [tsvilling], IPA [ʦvɪlɪŋ]. “Twin ache” would be Zwillingsschmerzen or maybe rather Zwillingsweh, which implies more of a yearning ache, compare with Heimweh “home-sickness”.
These are pronounced [tsvillings-shmayrtsen]/[tsvillings-veh], or in the international phonetic alphabet [ʦvɪlɪŋsʃmɛɐʦən]/[ʦvɪlɪŋsve:].
Doppelschmerzen would just translate as “double ache” or maybe rather “double pain”. As far as I know, neither Doppelschmerzen nor Zwillingsschmerzen or Zwillingsweh exist as compounds with a special meaning assigned to them. But Google tells me someone seems to have written a poem “Doppelschmerz” once: http://sammlungen.ulb.uni-muenster.de/hd/periodical/pageview/2357868.
If you are looking for special characters as the international phonetic symbols or Umlaut-vowels, this is a very useful website that lets you copy the symbols and gives you the unicode keyboard combinations, too: http://charmap.de/ .
If you want to know more about the international phonetic alphabet, here is a chart with sound samples: http://www.internationalphoneticalphabet.org/ipa-sounds/ipa-chart-with-sounds/ .
All the best,