After one spouse passed away - there is no "joint revocable trust" anymore...
The AB trust become irrevocable - and assets MUST be divided between A-trust and B-trust.
B-trust (sometimes called the Family Trust or Bypass Trust) becomes irrevocable.
and A-trust (the Survivor’s trust) will be used by the surviving spouse.
After assets are divided between A-trust and B-trust - these assets are owned by separate taxing entities - and may not be mixed.
So the By-Pass Trust is an irrevocable trust and is a separated legal and taxing entity.
As long as trust's documents allow - additional assets may be transferred to the trust - however for tax
purposes - such assets woudl be classified as gifted - and the basis for such assets woudl be the lesser of the donor's basis and FMV at the time of transfer.
That actually means - any "unrealized capital loss" will be "lost" if sold by the irrevocable trust - the gain or loss woudl be calculated using that basis - not donor's basis.
Sorry if you expected differently.