Just kidding. There's not enough piano memory. And this program won't solve the problem - on the contrary.
I like the YaS4 Bright Grand because it is so bright, yet rather mellow, and with the right reverb, it sound so beautifully wide and aerial...
But its bass are not very clear nor defined.
I like the bass of the Bosendorf better. It has a kind of macho sharpness which is just what I need - unfortunately its medium and hi-pitched notes are too thin to my taste.
NO PROBLEM : let's just split the keyboard, use the Bosendorf for the first two octaves, and the Yamaha for the rest !
In XL quality (I wouldn't go for less), that's about 400Mb for just one program, but hey, this is just the best piano sound to my taste ever. (I had to do away with the Italian Grand to make room, not without regrets though. I also had to get rid of all the upright pianos, which is too bad because I like the honky-tonk and saloon ones.
Life is the art of compromise, which may be why it so often sucks. 
Or maybe Clavia should just put more memory in their keyboards.)So, here it is. The Bosendorf should be in grand piano model #1, as from factory, and the Yamaha S4 in grand piano model #2. The split point is in 2nd split position. You can move it to 1st split position: that sounds good too. Any other place would defeat the whole point (and be noticeable), so better keep the split to either one of the first two positions.