Ayu Tsukimiya’s seiyuu, Yui Horie, is actually much more prominent in anime. She voiced Riki in Little Busters’ anime adaptation, and has also voiced Tsubasa Hanekawa from Monogatari, Minori in Toradora, and Felix in Re:Zero.
Also from Kanon there is Mai’s seiyuu who also voiced Mei Sunohara in Clannad, Michiru in Air, and Suzuha in Steins;Gate. And Sayuri’s seiyuu voiced Misuzu in Air.
Yukito’s voice actor in the Air VN also voiced Yoshino in Clannad, Kyousuke in Little Busters,and the protagonist of Saya no Uta,
Nagisa from Clannad is the voice of Otonashi’s sister in Angel Beats, and she did a few vocal songs for Symphonic Rain. Nagisa’s mother Sanae is also the voice of Uraha from Air
Masato is the voice of Ryuuya from Air, and apparently ever Fate Lancer there is. Komari’s VA has voiced faaaaarrr to much that she deserves. Most notably Principal Chizuru from Grisaia. Also from Grisaia, Yumiko has the voice of Kurugaya and Makina has the voice of Rin.
I’ll add to this if I come across any more interesting things