Totally exasperated with trying to make creme fraiche

I have posted before and got a LOT more information that I needed or could absorb, which may have complicated things.

My questions is simple and hopefully focused: I simply am trying to make creme fraiche using pastured (not UHT) cream and buttermilk that was cultured using one of the heirloom kits.

The maker of that kit - Cultures For Health -- it telling me that adding a buttermilk cultured by their kits to cream would simply create sour cram, not creme fraiche. They say creme fraiche needs a different culture.

I don't want to buy cultured buttermilk from the store to make the creme fraiche because it only comes in quarts and I only need two tablespoon to make the creme fraiche. This whole thing started because I am making butter and have plenty of uncultured buttermilk left over.

Can someone help. Please, if I may ask, I don't need the background on pasteurization and such, I just need to know the simple process of mixing in a cultured buttermilk using this heirloom kit. (Yes I know to leave it loosely covered on the counter for 24 horus and such.)

I am just stuck on why the cultured milk using this Cultures for Health kit would not work to culture the buttermilk and then mix with cream to make creme fraiche.