1)I have a meeting with a tax accountant at the same time.
2)That time overlaps with a meeting with a tax accountant.
かさなる:to overlap / to happen at the same time.
1) I have a meeting with a tax accountant at the same time.
"同じ時間に税理士とのミーティングがあります"
2)That time overlaps with a meeting with a tax accountant.
"その時間は税理士とのミーティングと重なっています"
I'm afraid I have another appointment at that time.
I'm sorry, but I have a prior engagement scheduled for that time.
その時間は税理士とのミーティングと重なっています.
I have a meeting with a tax accountant at that time.
その他の予定が重要かどうかという問題にならないように、直接に言わない方がいいと思います。「I'm afraid I have another appointment/a prior engagement at that time」は曖昧で便利です。
ビジネス場面で、さらに丁寧に話したい場合、「I'm afraid」や「I'm sorry, but」と言った方がいいです。