I replaced my single handle kitchen faucet to a double handle faucet. (No special reason.) I added a sprayer also. Everything has worked great, which has been about 3 months up until about a week ago. I have good pressure when I turn on either the cold or hot, but when I turn on the other, the pressure drops in half. The sprayer has great pressure all the time. I bought it at Lowe's but not sure what brand it is. Can you help me fix this problem? Thanks

The first thing I would do is open up the faucet and flush out the supplies. It sounds like some crud got in your system and got drawn up into the faucet.
I'll include a image of a two handle faucet for a guide. Good luck, Tom