For couples with taxable estates, "disclaimer trusts" are commonly used today to allow the surviving spouse greater flexibility in optimizing estate tax savings.
Here's how they work. Each spouse sets up their own revocable living trust. Husband and wife are co-trustees of his trust, using his social security number and, vice versa, they are both co-trustees of her trust with her social security number. Let's say husband dies first. His trust says "leave everything to my wife except that, whatever she disclaims, i.e. refuses to take, will remain in my trust". The disclaimer is a legal document that lists the assets disclaimed and their value. Wife remains as trustee on husband's trust after he dies and may use the funds in his trust for her health, maintenance and support. She may also remove 5% of the trust every year for any reason or $5,000, whichever is greater.
The reason wife is limited to health, maintenance and support is that if she had the right to take whatever she wanted at any time for any reason, the IRS would say that she had complete control of the funds and would then seek to tax those funds in her estate. The access for health, maintenance and support, however, is sufficiently broad so as not to cause a problem for her. She may also continue to buy, sell and trade assets in the husband's trust. This trust continues for her lifetime and pays out to the beneficiaries at her death along with her own trust.
Husband's social security number died with him so his trust took out a trust tax identification number (TTID) when he died and reported as a separate taxpayer during her lifetime. It is not includable in her estate. Indeed, what has happened is that husband's trust was settled on his death and left to his beneficiaries, but subject to wife's lifetime use and enjoyment of the trust assets.
The benefit of the disclaimer is that it allows the wife to decide (or the husband if wife dies first) how much to leave in the deceased spouse's trust based on her age, her health and the tax laws at that future time. On her death, his trust passes estate tax-free to the children, regardless of how much it may have grown.