From the NOTAM it looks like FISOs are being trained/ working on both the Tower and Approach frequencies - how does this work out as
AFAIK there is no other FIS unit which uses two.
The frequencies are "cross-coupled" so it doesn't matter which one you use, anybody on the other frequency can hear you. The same goes for aircraft transmissions.
Also is this going to be a long term thing and what's the effect on instrument traffic - are the FISOs being allowed to handle discreet 'company' instrument arrivals like Walney for instance?
NO! to both - and if I had my way it wouldn't even be going live at all