For employees, sponsorship qualification is determined by CURRENT annual income....not last year tax return returns. Current annual income is calculated as follows:
Employees- Latest pay period gross income multiplied times number of pay periods per 12 months.
Calculate that for each job and add together.
Self employed people- Determined by documentable income (last tax year Schedule C, Profit/Loss schedule, etc)
If you need a joint sponsor, I would opt for a single well-qualified joint sponsor. Good luck.