Which syllable is lengthened (stressed) in "asahi", as in Asahi Pentax?
If you're asking about how the Japanese pronounce this, the correct answer is there is no syllable stress. Japanese does not have syllable stress the way many Western languages do. In Japanese, sometimes stress is indicated by lengthening a syllable, but there is no syllable lengthening in a-sa-hi. So, a-'sa-hi is a Western pronunciation, but incorrect. So is 'mi-tsu-'bi-shi or ka-wa-'sa-ki. One thing I have noticed with the Japanese pronunciation of multisyllable words, such as Mitsubishi and Kawasaki and kamikaze, is a slight rise in pitch on the second syllable. But I don't hear a similar rise in pitch on three-syllable words, such as Asahi or Yamaha or Suzuki or sashimi. But I would hesitate to call this slight rise in pitch stress. It just seems to be more of a feature of multisyllabic words of four or more syllables.
I think it bears repeating an earlier comment I made in that, just because a syllable is geminated, this does not necessarily indicate stress. It's just the way it is. Take Tokyo or Kyoto or Osaka, for example. All three of these cities have geminated syllables. Tokyo: とうきょう, Kyoto: きょうと, Osaka: おおさか. Curiously, Osaka's gemination is a repetition of the お (/o/) character, instead of the more typical おう spelling. Perhaps this is because of the fact that it's a gemination of the pure vowel, rather than a consonant-vowel combination? I dunno.