I can't seem to find the reference for this tax question:

Husband and wife jointly held a brokerage account titled in the name of their family trust. The husband died in May and the trust directs that half the assets go into the A trust and the other half to a bypass trust that will not be included in the survivor's estate when she dies.

Question? At what level can we split the assets for the bypass trust.

Example: they owned two stock positions 100 shares of ABC and 100 shares of XYZ. Does he own 50 shares of each and we can, therefore, step up the basis of his 50 shares and put them in the bypass. Or, does he own 50% of each share and therefore all shares have their basis adjusted to reflect the average of the original basis and date of death value. Then 50% of each position goes to the bypass.

Is this in the IRC somewhere??

Thanks

Kelly