Once the data is in SSAS, the most granular you can get is at the dimension keys. As mentioned by Steve, to be able to analyze the two records as separate, you'll need another dimension that separates them. SSAS is meant to be used as a tool to perform aggregate data analysis so when it stores data, it loses all the record level details.
Creating a SalesID/TransactionID that uniquely identifies the record and creating a dimension on it will allow you to perform the calculations you want, but you'll face a large storage and performance cost since you'll no longer be only storing aggregate data in SSAS.
If you need to perform standard deviation and percentiles across each individual record, you'll probably just have to do it in SQL.
I've had implementations where the user wanted to slice and dice on SSAS in excel, then drill into details which were queried from SQL. I used ASSP
to tie an action to run a stored procedure that would return a data set based on parameters passed by the excel slice.