You are not permitted to may your IRA contribution and your wife's IRA contribution to the same IRA, so I hope that not what you meant when you said that.
Only the first $2,000 of each individual's retirement contributions are considered for the credit. If you and your wife each made IRA contributions of at least $2,000 each and neither of you has made any recent retirement distribution, your AGI apparently puts you and your wife in the AGI range where the credit is 10% of $2,000 = $200 for each of you for a total of $400.
