Which one to install first?

Doesn't matter one bit.

Well maybe a tiny bit. I prefer to start with the rear as if I mess that up odds are that those cables can be used at the front. But if the front is cut too short, only place where that can be used is on another bike.

dabac said:

Doesn't matter one bit. Well maybe a tiny bit. I prefer to start with the rear as if I mess that up odds are that those cables can be used at the front. But if the front is cut too short, only place where that can be used is on another bike.

Thanks for your advice. I watched lots of YouTube that teaches how to adjust derailleur WHILE one of them is working, that's why I asked the question because I'll need to remove both broken cables first. Which means I'll have to pick one (either front or back) to work with first without the other installed. I'm afraid after I setup.... say... the rear with all adjustment done, then after I install the front, it will mess up the rear setting, or vice versa. Just don't know if there's a priority (like setting up a new bike from scratch) of what to do first, that's all. Anyway, thank you again and I will do as you said, start from the rear.

Rear shifting and braking is entirely separated from front shifting and braking, so don't worry about it.