IF(
    AND(
        npo02__OppAmountThisYearHH__c > npo02__OppAmountLastYear__c,npo02__OppAmountLastYear__c > 0
    ),
    'Thank you for increasing your giving.','FALSE TEXT'
)

IF(
    npo02__OppAmountThisYearHH__c > npo02__OppAmountLastYear__c && npo02__OppAmountLastYear__c > 0,
    'Thank you for increasing your giving.','FALSE TEXT'
)