It's not possible to merge two survey designs or the results of two separate surveys.
Instead, create multiple collectors for one survey. The results for each collector are merged in the Analyze Results section of the survey so you can easily view and export all survey results.
It's not possible to merge the results of two separate surveys, even if they are identical in design or copies of one another. If you need to merge the data of two separate surveys, export the data for each survey and try combining the files in Excel.