Latching relay dual supply to coil under fault conditoin
The manufacturer's page gives minimal technical information and given the evident significance of the application and the intended use of the relays then a query to the manufacturer looks to be by far the most reasonable solution. I may be wrong :-).

Data sheet of sorts here - page 12, second column. No information on dual activation. 

You cannot guarantee any result if you present conditions not defined by the data sheet.  

EITHER your answer is in the data sheet
OR we, like you, can only speculate.  

I would EXPECT that if both conditions were applied then the relay would "most likely" remain in its last state while both remained applied - BUT this is uncertain. 

It is more certain but not certainly certain that if one remained applied for longer than the other by more than the maximum time to operate/release that it would produce the final result
BUT it is possible that the dual application MIGHT put the device into an undefined state that locked it. 

The safest approach is to build a circuit that operates in a fail safe manner such that dual application is not possible.  This is harder than not doing it but hardly hard. 
You can then decide what action you desire in any combination of dual coil activation attempts.

Examples only:  


If a secondary activation attempt occurred during an initial one the secondary activation should be ignored.
or  
If a Reset and Set occur simultaneously then the Reset should be accepted. 


Note - you may need to add eg


If a Reset and Set occur simultaneously and the Set persists for longer than the Reset then the Set should be ignored until it is deasserted.


There are likely to be other "maybes" which you meed to examine the implications of. 




  The problem i have is that i am having to do the whole circuit via relay logic rather than plc, i have a number of interlock auxiliary contacts in the circuit and the circuit is controlled by sprung load rotary switch. But I was more curious than concerned about the circuit design as it will always fail safe.


Depending on how important it is to you, you could eg lockout the set coil when reset_in is asserted, using a relay to do it.   

For "extra points you could have a relay which continues to lock out access to the set coil for as long as set_in remains asserted if set_in is ever asserted when reset_in is asserted. May take 2 or 3 extra relays - only worthwhile if the result is important to you. Without such, if set and reset are asserted together you'd expect the last man standing would win.