Yes, that's right I would like to replace my flexible line with a copper line? How easy would this be and is it possible? Reason I need to install a saddle valve to hook up to a ro/di unit.

Replacing flexible supply line with hard copper supply not difficult. Most difficult thing will probably be disconnecting and connecting supply to faucet up behind sink. Don't think saddle valve will work on 3/8 supply tubing however.

Better to replace stop valve with a stop valve that has two outlets.

I agree with Herold, you can replace your angle valve with two out lets see the image below, thanks.

John