Hi and welcome back!
Here is extract from IRS publication 504 that addresses your situation - http://www.irs.gov/pub/irs-pdf/p504.pdf
Joint estimated tax payments. If you and your spouse made joint estimated tax payments for 2010 but file separate returns, either of you can claim all of your payments, or you can divide them in any way on which you both agree. If you cannot agree, the estimated tax you can claim equals the total estimated tax paid times the tax shown on your separate return for 2010, divided by the total of the tax shown on your 2010 return and your spouse's 2010 return.
If you claim any of the payments on your tax return, enter your spouse's or former spouse's social security number in the space provided on the front of Form 1040 or Form 1040A. If you were divorced and remarried in 2010, enter your present spouse's social security number in that space and enter your former spouse's social security number, followed by “DIV” to the left of Form 1040, line 62, or Form 1040A, line 39.
So - first of all - there should be an agreement or a court order how estimated taxes should be divided. If there is no agreement - follow the procedure above - you may not simply use 1/2 of the estimated tax.
thanks very much