PlanGuru Excel import utility requires that you to load balances for your entire chart of accounts from a single import sheet. As a result the income statement and balance sheet must be imported together.
If you try to import them separately you will encounter issues. For example, if you load the income statement first and then the balance sheet after, the income statement will load fine, however the action of loading the balance sheet will "zero out" the income statement.
PlanGuru functions this way both for practical and technical reasons. Forcing user to load the entire chart of accounts at once prevents confusion that may result from multiple and incremental imports.