For version 9, you can still download the data directly from the Admin module in a .csv file. Bring down the two .csv datasets, open them in Excel or a text editor, and copy the one into the other. (Create backups just in case!) Then import the combined dataset into one of the two surveys. One tip: If you go into the Advanced tab on the Survey Settings, you can tell the software to start one survey's respondent numbering at, say, 10001 so that there are not conflicting respondent numbers when you merge the two data sets.