Hello, is it possible to get the144 hour China transit visa twice during consecutive weeks?

Yes, this is definitely possible and your suggested itineraries are definitely eligible for 144 hour visa free transit twice. There is no limit on how many times you can use this visa free transit scheme. Just one remark... If your stay in Xiamen is shorter than 24 hours, then you will be eligible for 24 hour visa free transit and not 144 visa free transit. If you can, it is always advisable to extend your stay for at least a minute longer than 24 hours in order to enjoy 144 hour visa free transit.