If you did not send the tax transcript to begin with when you submitted the I-864EZ, then that is the entire problem. The I-864EZ replies on the income from your most recent taxes, so if the tax return documentation is not submitted with it, then they will need to request it and they will not know if the sponsor meets the income requirement or not.
Just so you know, the immigrant would never be their own joint sponsor. An immigrant's income can be used if you provide proof it will continue from the same source after you become a permanent resident, but it is added directly to your USC spouse/sponsor's I-864. The I-864EZ could not be used in that instance.
