You need a new joint sponsor with a higher income. “Co sponsoring” is not an option .. Only when the petitioner is using household members income to add to their income.
The other alternative is that the petitioner relocate to the US 4-6 months prior to the interview, secure s job that meets “ current income” requirements and complete a new I 864 with documentation for the beneficiary to take to the interview.
Please fill out your visa type and country details and your timeline, as these details are critical to getting the best and most accurate replies.