I have merged several credit cards from the same bank into a single card in the past. It was always done instantly and the merged card had the combined credit limit of both. This was the story, atleast for Citibank, Chase and as far as I remember, Bank of America.
However, with HSBC, it is different. Here is what the Cust Service rep had to say:
“It will take 10 business days to process this transaction and you are not guaranteed to have the combined limit of the cards after the merger”