Ok you could make a turbo using your 2 units but you would need to match the compressor wheel to the right housing so let's say you put the compressor wheel from your t25 on the t2 turbo you would need to have the unit balanced to be able to run without it shatting it's pants in no time at all. Oh and while it's being balanced best have the bearings replaces as its already apart. Then you would also have to make sure the compressor housing fits too so you might need a different backing plate to make it fit.
The cost involved in doing that is not really worth the gain best off buying a good second hand one if you can find one which you know is good
trying Romil's turbo and finding it works fine and is good money might be a better option
Also could be a few reasons why you t25 isnt holding boost
Is the actuator working how it should
Is the turbine housing cracked so it's bypassing the turbine
Are the bearings ok as the wheels could be catching on the housings
Is your manifold leaking making the exhaust gases bypass the turbo
long shot but you never know lol
Eerrrrrmmmm that's all I can think of but I'm sure there are more reasons for it