I have a client who had a corporation, went to sole proprietor on the advice of a guy who claimed to be an accountant, then re-incorporated on the advice of a real accountant. Plan documents were done to reflect all of this.

For 2004, I have participants with 2 W-2s, one with sole p EIN and one with corp EIN. Add them together? All but 1 are relatives. (Owner will have little-to-no sole p income but W2 from the corp.)

If all of the entities are participating employers, add them together. If the Sole Prop is taking the deduction make sure you reduce the income, also if using a software, make sure it is aware of the SS withholding on the W2's.