[Cedarwinds Farm]

My first thought is that there might be something going on with your milking machine. If I have off-tasting milk, my first move is usually to go over my entire milking procedure step by step, and make sure everything is clean and the milk is getting chilled quickly. Then I start looking at the does' health and mineral intake, and other environmental factors.
One time, the thermostat on my fridge got bumped up a little bit, so the milk wasn't staying cold. Now, I keep a thermometer in there and check it every time I open the fridge. Little things like that can make a big difference.

 

[Cedarwinds Farm]

Would a copper deficiency make a difference in taste? I have also heard people say they give a half of a fig newton to help make milk sweeter.

Never heard of the fig newtons! Copper definitely affects flavor, as does any other mineral deficiency, stress, parasites, heat cycles...if they have access to pasture, they could be eating something that will flavor the milk. Mastitis will cause an off flavor, too.
If it was me, I'd hand milk each of them and keep their milk separate. That would tell me if one doe has an off taste or both, and if neither has it after hand milking, then the issue is with the machine.
I've never used a milking machine, but I know they have to be kept really clean and I also know there are rubber parts that can age and crack, harboring bacteria. You probably know way more about that than I do, as you're actually using a machine.

 

[Cedarwinds Farm]

Hope you can figure out what's going on! There are so many factors at play in milk flavor, sometimes it feels like you're playing detective.
And welcome to TGS!

I will try that! Thank You!