"Can he continue to work for his original company 'A'?"
答案是YES,但沒有說的是你必須重新申請TRANSFER,因為你是用的同一個名額.如果你是分別用的兩個名額當然不用擔心.
"A person can have multiple H1, but can work for only one company at a time" 是錯誤的.一個人可以同時有兩個H-1,一個為A公司FULL TIME工作,一個為B公司PART TIME工作.