There is no common practice of two layers of silicone. Silicone just plainly doesn't stick to silicone, if it did, we wouldn't have to remove the old stuff first.

The rest, it's difficult to fully understand without a photo.

Photo?

This is what I used Super easy, you can watch the stuff expand out the top of the foundation wall on the outside as proof it makes it all the way through.

Fix your grading to prevent water flowing towards to the house.

Take any small motor ( old record player ), and watch this video https://www.youtube.com/watch?v=ZF6hTXecPLw

I think you will be able to put the two together and make what you need.