Can I used the 72hour/144 hour visa exemption multiple times.

Yes, you definitely can. There is no limit on how many visa free transit schemes you can use within certain time frame... In reality you can use this option literally on daily basis.

In other words your itineraries:

Kuala Lumpur - Xi'an - Hong Kong

Hong Kong - Shanghai - Bangkok

Can be done in a sequence. You should just take care that you always follow the very basic rule and that is that the immediate points before and after your mainland Chinese destination have to be in two different countries or eligible territories (eligible territories: Hong Kong, Taiwan and Macau). Also, you are not allowed to have two mainland Chinese stops in a row, for example:

Kuala Lumpur - Guagzhou - Xi'an - Hong Kong

Only one Chinese destination per one 144 hour visa free transit. Also, it is 144 hour visa free transit in both Xi'an and Shanghai.