If both husband and wife are US citizen, their estate tax exemption are 22.36 mil, and the earlier deceased spouse's exemption is passed onto the later deceased spouse's exemption, so that no matter who dies first or last, their estate won't be taxed for property under 22.36 mil. This is called portability of estate tax exemption.
If one spouse is not US citizien, things get a little complicated.
Hope this answers your question.
