yes, it stacks, what does obviously not stack is stealth given by nightfighting + stealth given by night shields, as its the same USR.
I guess its useful for infantry models without transport... if we used any. Venoms already have a 5+ invulnerable save and raiders/ravagers will probably want to jink to get that sweet 3+ cover save thanks to night fighting/night shields.
Not a very useful rule tbh.
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