Many couples sever joint tenancy on their property to then own as 'Tenants in Common'.
Imagine a red line drawn down the middle of the property, you own half, your partner owns their half.
You each make a Will leaving your half of the property in trust e.g. for children, this may be to protect their interests e.g. 2nd marriage or means tested benefits in later life.
On 1st death, half the property is now in trust with the surviving partner still owns their half and has a lifetime interest in the 1st half by being able to still live in it, sell up and move elsewhere etc.
On 2nd death, the whole property can then be sold. At this point the original half of the property (1st death in trust) may be subject to capital gains tax but is outside the 2nd death estate for inheritance tax.