I split community property w-2 and 1099 income and withholding 50% on each return. Form 8958 gets populated with those amounts, all under the column for the one you are doing the tax return for, but you must manually add the other 50% into the other column for the other spouse/partner, so that the total column comes up with the real totals.
For any separate income, like pensions, social security, or anything else separate, you enter the whole amount, but you must show it on the form 8958 with 100% allocation to the one spouse/partner, and 0 tot he other.
In other words, based on what I saw my ex-CPA do, all incomes must be shown on 8958 so that you are disclosing everything.
Also, my ex-CPA used to attach a copy of my domestic partner certificate, so I did that also.
