Welp, I can't think of a drug that would be worse to combine with SN than datura.
How will it affect the plan? Well it shouldn't have any bearing on lethality of the SN in theory. However, vomiting is extremely common on datura, so it could affect the absorption of the SN depending on the timing of it all. However, it will affect things by making the timing of your dosing more confusing, and by making the experience horrific.
Datura is an extremely powerful deliriant drug. It produces intense (generally unpleasant or worse) delirium that lasts about 18-50 hours. It produces a number of side effects in addition to vomiting that would be extremely unpleasant to experience in addition to the side effects of SN. Things like extreme dry mouth, extreme confusion and delirium, agitation, fear, and anxiety, tachycardia, hyperthermia, seizures, etc.
You said "like (datura) also kills people I have heard," to which I would tell you that YES it is possible for datura to be lethal, but not reliably so and not due to any mechanism that would aid the SN.
Planning to "be high" on datura prior to dosing the SN is just absolutely silly to me, as you have absolutely no idea how a drug like datura will affect you, your motivations, your motor coordination etc, and attempting to dose the SN and datura so that they kick in at the same time would be unreliable due to Datura's onset having such a wide potential range and the chance of it inducing vomitting is extremely high... and also... just why?