take the time it takes both together as 'a'

now,

machine x alone takes 'x' hrs

machine y alone takes 'y' hrs

you have to find y - x

rate X time = work

(1/x)a + (1/y)a = 1.............'standard' work rate equation

a = (2/3)x.........................deduced from statement I

2/3 + 2x/3y = 1..................substituting

2x/3y = 1/3

2x = y

This gives us x and y in terms of each other, however no constant term is there to find the difference between 'x' and 'y'....

Hence I is insufficient

Also, stmnt II gives us the same info, albeit written in different words..

Hence, imo E


Note :- This is the algebraic solution, and I must say, a 'safe' bet...

However, if you want to solve it quicker, just have a look at both the statements, none of them gives a constant, to signify the difference between the hours, both the statements just give a relation between the variables....hence the answer cannot be determined from either of the statements....Hence E.
However, i would not advise you to adopt this strategy until you have a very good grasp of these kind of problems.