tough to explain this one, but it has to do with a grammatical form called 'identifying numbers/ordinals' in Arabic (tamyiiz 2al3adadd)
If عند is in the beginning it does not change form.
If عند comes later in the sentence (better form) then it conjugates with the owner.
عند تامر وكاري بيت
تامر وكاري عندهما بيت
It (the sukoon) is not mandatory, but its presence sure helps when you are learning. When the long vowel ي is preceded by a fatHa and followed by a sukoon, the sound formed is the diphthong "aye" which is why the Arabic word for house is pronounced "bayt" and not "beet." The sukoon also means that the syllable ends with the consonant above which it is placed. I have read it described as the absence of a vowel or as the "silent vowel." When placed after (above) a و preceded by a fatHa, the three together sound like "ow" instead of the "oo" sound of the long vowel و by itself.