Contrary to your defense of them... The voicing acting isn't believable, the audio quality for the voice recordings is not the greatest, and the voices aren't remotely fitting to the models. Sayako especially sounds like a young girl instead of an adult. I get the mental image of a 14 year old listening to her voice, which is not really appealing. XD
I won't harp on the big boobs, but the outfits are pretty ridiculous. No one is legally going to walk around in the outfit you have Tara in, she would be arrested for indecent exposure. Which further sets up the "this is just going to be a porn" feeling, which completely conflicts with the score/music.
The expressions, especially on Sayako, are well done, and the small amounts of animation are fairly well done and fluid. I don't know how much credit to give you for the renderings though since you admitted to using bases, and I didn't see, or bother to look hard enough for your links to your original sources. >)
But the concept itself is not original or appealing.
As I stated I can't get over how inappropriate the music was for the type of teaser this is supposed to be. The music implies something epic is about to happen - and I don't mean epic as in girl-on-girl sex. Epic as in we're about to go on a timeless quest to save the universe. Which I'm pretty sure they aren't doing.
Overall I can't give this many stars. I like hentai fine when it's done well and classy - but this does not come across as classy. The faces and hair on the models are beautiful, but the way the boobs and butts are done makes them look like ducks when seen from the side. I'm not asking you to change it, just stating it doesn't mesh with my aesthetics. I like sexy women and large breasts are fine, but they just look uncomfortable.
Change up the music sequence, change up the acting (either get the girls to sound less.. girly.. or change VOs entirely), and this would improve in my opinion.
(Also, I sincerely hope your VAs are both over 18, Sayako especially didn't sound like it. >w<)