As I understand the law I can best explain by giving examples. I am assuming the couple in my example are not married because of complexities of IHT if married.
Joint Ownership
A & B own a house worth £500,000. A dies, their half of the house automatically becomes the property of the other joint owner. Their estate is nil. no IHT. B then dies. Their estate is worth £500,000, above the IHT exemption amount so tax will be paid
Tenants in Common
House details as before. A dies, his share of the value of the house, £250,000, forms part of his estate for tax. but because it is below the level for IHT, no tax is paid. B dies,now their estate is now only half of the value of the house, £250,000, This is below the IHT level so no tax is paid.
In the second example, A's estate is usually put into a trust for the benefit of family but giving the survivor a life-tenancy of the other half of the propeerty, icluding allowing it to be sold to buy a smaller property etc.
I think the usual thing to do is set up a Discretionary trust where the family are beneficiaries and trustees and the trustees can choose to give money to beneficiaries as they wish so that if someone goes into care, the house can be sold and when the survivors share of the cash is used up in care fees they can be given money from the trust.
However I am a not a lawyer, although I have studied law, and the law on trusts has been changed recently, but this is how I understand the law of ownership and trusts to work.