Is it possible to sequentially mount twin turbos on a 2.5 without disastrous consequences for the engine or wallet? Thanks for the time.

 

Hasn't been done on the Lima 2.5l as far as I know, and its 1990's technology, Toyota did it on Supra models in late 1990's, 3.0l inline 24v 6 cylinder

You use a smaller turbo at lower RPMs for better throttle response, all the exhaust runs thru small turbo at this time, then at mid RPMs some of the exhaust is sent to larger turbo to get it turning, then at high RPMs all exhaust goes to larger turbo for full boost
It gives faster throttle response with smaller turbo, then higher boost with larger turbo

There have also been parallel twin turbos on inline engines, usually 6 cylinder, 3 cylinders power 1 turbo and 3 cylinders power the other one, so like a dual exhaust system on an inline engine
Benefit is smaller turbos with quicker response so less turbo lag than 1 larger turbo

Found this, good read: https://en.wikipedia.org/wiki/Twin-turbo

Smaller turbos have faster response and lower boost
Larger turbos have slower response and higher boost

The problem with 2 turbos on the one exhaust pipe is that the second turbo can't add as much boost as the first so might as well not be there
i.e. if first turbo is at 8psi boost then second may be at 6psi, so 8psi is the boost, 6psi doesn't add anything
You could lower first turbos boost down to 6psi with a wastegate but what have you gained.................nada

 

So then a small turbo would work better? Would I need to change out anything inside my 2.5 to allow it to work?

 

2.5l Lima engine is rated for 120 horse power
So all the parts inside are engineered to withstand the stress of 120HP

You want to increase the HP so..................what do you think may happen?

Yes, eventually a part will get over stressed and......................well, undetermined results, depends what breaks

Some say these Lima engines can stand 7psi of boost and hold together, I would check the Mustang forums, they have more info on that, 2.3l Lima is the same engine
But the engine you have now is rated for 120HP so technically thats its limit, no one can say what will happen at 121HP and up on YOUR specific engine, lol.

Pro engine builders do the math, spend thousands and STILL blow engines, so use your best guess after reading a bit more over at the Mustang sites