I think you should clarify how you are using the concept of ‘same itinerary’. Presumably, it means flying the same flights, and not on the same PNR, since you can’t have a BE and non-BE ticket on the same PNR.

Technicalities aside, as already mentioned, BE doesn’t get access to economy plus - at least, they can’t buy a seat assignment there - only UA can assign BE pax to E+, and typically, that will happen only if those are the only available seats left (and I personally think UA should be moving non-premier, non-BE flyers for free to E+ rather than BE flyers, elite or not, but that’s probably fodder for another thread). Even if that happens, very unlikely to be seated next to you - the computer will pick the seat, not the pax. While there have been reports of the kiosk allowing some BE flyers to select E+ at check in, I believe it’s been limited to premiers.

if someone wants to sit in E+, buying a BE ticket is not a good way to accomplish that without a lot of luck.