Two server powersupplies in a row dangerous?
Question:
Does this modification disable safety features?

NO.  Not if done exactly as stated.   

It enables a status resistor loop to enable the supply and only removes the secondary earth bond and not the primary AC safety ground to enable series DC connection of 12+12=24V @ 75A  (850W at 120Vac, 1200W out at 230Vac rating high efficiency) making the two supplies unique with an unmodified one = Earth gnd = V- = 0V and modified one 12Vout  to V- to 24V=V+. with AWG 8 or heavier cable.  You can choose an external fuse for the charger cables you select if undersized. ( e.g. or use welding cable)

I understand all the safety requirements and fundamentals of this category of designs and none of the safety features are bypassed or disabled in the article link below. However, any modifications are not permitted for resale, but ok for personal use.

I assume this is the one you have read, (because it is written by a professional and popular with hobbyists. ) 

But if you make any mistakes with your fingers touching charged HVDC contact points  inside, this is your doing.  -48V PSU's are normally designed with an earth ground jumper option for V+ or V- earthing.  But for Server communication V- is always earth grounded for noise immunity.  Short,  #8 or bigger solid or stranded welding wire for interconnect cables are necessary to prevent short circuit wire fire.  Your application load needs some kind of soft start switch to prevent overcurrent trips then a bypass series load for high current.  How you regulate your battery charger with a BMS is outside the scope of this question.

DO NOT attempt anything described herein without ensuring that you have taken all the necessary precautions to safeguard yourself and others from injury.