Could you link to data in one spreadsheet from another using Numbers for iPad?
No - there isn't an URL scheme or addressing scheme to make that sort of link.
This isn't something that even works on the Mac version of Numbers so at least there is parity across the OS in this case.
Just digging into Numbers on iPad myself. A very useful way spreadsheets can be linked is via cell references. Perhaps this answers your question.
I see the following in the Numbers documentation, in the section Data and functions: Types of arguments and values:
"If a reference is to a cell in another table, the reference must contain the name of the table... For example, [in Table 1 a cell could have a formula]
"If the reference is to a cell in a table in another sheet, the sheet name must also be included ...
This is familiar to users of Excel, as far as it goes; I see now that it only works within files and not between them.