Your lamb could not eat feed or loose minerals meant for goats. Goats have a much higher nutritional need for copper. The levels a goat needs will give a sheep copper toxicity.
On the flip side, if a goat is fed sheep feed, then the goat will become deficient in copper. So if you can keep them separate at feeding time, maybe top dressing the goat's feed with goat loose minerals, and offer them all sheep minerals free choice, it could work out. Keep an eye out for signs of copper deficiency in the goats. One of the most obvious signs is (if you have one) a black colored goat turning rusty, because copper is a critical element for the creation of black pigment. There are many other signs of a deficiency, and this is a great article for it, too.
I personally struggle with low copper issues in my herd. The reason is the ultra high levels of iron in the water (with towns nearby named Iron Mountain, Iron River, Ironwood, etc. it is not surprising). High amounts of iron inhibits proper copper absorption. i have to give my goats a copper bolus (copper oxide capsule) on a bi monthly basis. If I miss a month, my black goat, Suzie Q, starts turning rusty very quickly.
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