It can't be guaranteed to work. Depending on the object, transport Y may be a kind of "delta" of what you put into X.
For example, with ordinary abap programs, the first transport contains
R3TR PROG z_myprogram
This transports the program with all texts and includes.
If I then change the TOP include and save against another transport, that will only contain
LIMU REPS z_myprogram_top
BW objects are rather more complicated, so the risk is even higher that just taking the last transport won't work.