Don't put or attach any financial information in or to your introductory or sponsorship letters. Chances are it won't be taken into consideration (wrong place). Attach it to your financial documents or to Appendix 2. Yes, it's a good idea, esp if your situation is a bit more complicated, like several jobs, combining two or more sources etc.
You mention your second job under 'Other information' on Appendix 2. And put the total amount from two jobs on the form itself under the question 'what is your total income during the last 6 months?'
You can just attach a very simple and clear statement of how you are meeting the requirement on a separate paper.