I have two rental incomes attached to a Property, but what I add an action to sell the property I can only choose one income. Is this right?

Yes, currently when the Action 'Sell a property' is chosen there is the option to choose whether an income is going to stop on this sale.  

This will be a change in a future release, but for now we suggest adding just one Rental income record with a combined amount,  and potentially the same with the debt. 

The other option if the potential date of sale is known, is to record the additional income/debt records with an end date of this potential date.