I’ve been in that end and I’m not sure how they can split it between both sets of supporters.
Just build a much bigger and a permanent stand behind that goal and give the whole stand to away fans for league and cup games. Even a bigger temporary stand like the one at Gillingham but with a permanent roof structure around it would suffice.
Bournemouth lease the Vitality Stadium and there are currently 5 years left on the deal. This complicates any work on the existing site.
They are building a new training ground on a 56 acre site elsewhere so watch this space.