Heaven (or fate) does not grant two exceptional gifts to the same person; a proverb meaning people rarely possess two outstanding qualities at once.
彼はスポーツは得意だが勉強は苦手だ。天は二物を与えずというわけだ。
He's good at sports but poor at studying. I guess heaven doesn't grant two gifts.
あの俳優はイケメンで演技もうまいけど、天は二物を与えずは当てはまらないね。
That actor is handsome and a good actor—'heaven doesn't grant two gifts' doesn't apply here.
どんなに才能があっても苦手なことがある。天は二物を与えずという考え方も一理ある。
No matter how talented someone is, there are things they can't do. The idea that heaven doesn't give two gifts makes sense.