Dell 2950 III Processor compatibilty. - two different processors

Hi all.

I have dell 2950 with stock e5320.

I added E5440 to second slot but dell showing error.

So i put E5440 to first stot and remove E5320.

Now i think to buy second processor L5420

Can this server work with

E5440 in 1slot

L5420 in 2slot.

Or both processor need have same version (like 2x E5440).
Or i can mix two processors?

I suppose that e5450  can't  work with e5320 by fsb (1333 vs 1066)