Could members please provide some feedback about concerns on stacked shell and tube heat exchangers (fixed tubesheet design). Current design propose two 7 meters BEM exchangers

I have seen one and heard of another failure of these stacked types and wnated to gather more information. My concerns are as follows:

1. Additional stresses imposed by differential thermal expansion between the two shells (these exchangers are in series and it is possible in the long run)
2. Fabrication tolerances, misalingment of flange, levelness of saddle imposing loads that has not been considered in the design
3. The excahngers needs to be tested together including demonstration of leak tightness.
4. These exchangers could not be delivered stacked, because of the size and transportation issues. The need to be re-aligned and installed at site.

Please review the above, share any other concerns and past experience.

Thanks in advance.