I have always just put the big one in the back, and the smaller one on the right rear fender. Both as low as possible. I will never put these monstrosities on the front.That does not look like the correct place for the current stickers.
Clean Air Vehicle Decals for Using Carpool and HOV Lanes - California DMV says that the big ones go on the rear sides, while the small ones go on the front bumper.
I don't think I have heard of anyone who was fined for not having the stickers in the correct place. Cops have far better things to do than pull someone over for this. I think as long as they see that you have a sticker, they will leave you alone.