Yes, continue depreciating your 1/3, and yes, ALL assets need another 'duplicate' asset to depreciate the new portion.
For the 'new' assets, I'm not 100% sure if what you are saying is correct, so let me rephrase it another way. Each 'new' asset will have 1/6 of the Fair Market Value at the date of death, and yes, it starts on the Date of Death and goes for the next 27.5 years. It 1/6 is because the old owner had 1/3, and that gets 'stepped up' to FMV. Then that 1/3 is split between the two of you, so you each use 1/6 of the FMV at Date of Death.
Does that make sense?
