You will need to create 2 asset records for the single asset:
20% of the cost of the asset: depreciate this at 100% CP.
80% of the cost of the asset: depreciate this at the applicable rate.
When the asset is sold, both assets need to be sold, where the sale price is:
Less than Book Value of the 80%: sell the 80% at the sale price, sell the 20% at 0.
Between the BV of 80% and the 80% cost: sell the 80% at the sale price, sell the 20% at 0.
Between the cost price of 80% and the cost: sell the 80% at the cost price, sell the 20% for the remainder of the Sale price.
Greater than cost price: sell the 80% at cost price, sell the 20% for the remainder of the Sale price.