5* Solo Rate up or multiple Rate up

They'll each have their own individual banners, in addition to a shared banner, depending on the day. 4-star Nero will be alongside each one of them as well.

With brynhild and Nero both on rate up, the chance of getting either of them is 0.4% each so if you don’t mind getting either one, you’ll have a 0.8% chance of getting someone you want.
If you pull during a solo rate up it is 0.7%.

Ok, here is the thing: your chance of getting either of them is 1%, no matter if there is single rate up or multiple rate up! If you want both of them then the chance is even lower. Rate up for one 5* servant means than in the 1% chance of getting a 5* servant, 70% of him/her being the servant on rate up. Multiple rate up doesnt mean that your chance of getting them is higher, it just means that you will likely to have that servant if you manage to get that 1% chance to proc. To answer your question, your chance of getting Nero/Brynhild is the same in both cases, but it is recommended to roll for multiple one if you want them regardless.