Frankly, @TexasTea, if I were going to mail the returns, I would use two separate envelopes so there is no question that two returns were filed (obviously, use a method whereby you can get some sort of proof of mailing and delivery).
As far as "connecting the dots", the IRS will likely do little, or most likely nothing, prior to exceeding the lifetime exemption or at such time as the donor is deceased and a final return is filed (and an estate tax return, at such time when a 706 is filed they tend to pull all previously filed 709s).
