With Multiple Record Layout or 2 Sided Layout, you can have one or the other, but not both.
Here is one workaround that requires modifying the data source.
Create your 2 up layout in Indesign, this can be front and back (2 Sided Layout)
Your data probaly looks like this:
Name Address John Smith 123 Main Street Susan Brown 456 Oak Avenue Mike Howard 23 Union Ave Kathy Williams 56 North Street
Modify the data to look like this:
Name1 Address1 Name2 Address 2 John Smith 123 Main Street Susan Brown 456 Oak Avenue Mike Howard 23 Union Ave Kathy Williams 56 North Street
Insert your placeholders into appropriate position, and complete data merge
Thank Jeffrey, it was called "Work & Tumble" back in the offset days, I totally forgot about that.
If your 2up is "1 front" and "1 back" on the same side of the sheet, then you don't need to modify the data. But, you still need to create the layout as such before running a data merge.
The suggestion above is for a 2up layout for 2x fronts on one side and 2x backs on the other.