MacBook air won't start unless I hold the power button for at least 5s

"You stated you have two MacBook Air's of the same series and you where using one for the parts to fix the other. Are you sure the parts are good from the parts system? It sounds like you have two systems with the same exact problem!

As you talk about a chip which exploded we really need to know what that chip was. As it could explain why you're having problems across both systems.

So what would I do here??

If you can put all of the original parts back together with each system. Hopefully you took the time to mark the parts or take note which part came from each system. Once you have everything back as it was built we can try to figure things out. But, this will take some time and you need to fully answer our questions. It still will likely need some parts so it may not be the best way to get this system going. And besides, you still have a second system to get working right?

Or, you might want to cut to the chase here! Just visit an Apple Store or Apple authorized service center to let them dig you out of this mess. Ask them for the bench fee plus parts service hopefully it won't be to big a hit."