Can k render be 're render over the top

Anything can be plastered over providing you have an adequate key. Assuming the existing render is solid then you need to either form a mechanical key which could be grinding cuts into the old render giving the new render something to hold onto, or using some form adhesive like SBR to glue the render on. Personally I would consider painting both walls, much less work and probably a lot cheaper.

As above painting is the cheapest option, just check with the paint manufacturer that the paint is suitable to go over monocouche render. With regards to re - rendering it would need to be a specialist system not sand and cement. It would be best to find out if the K-Rend is silicone or mineral based and plan a new render system based on your findings. You would probably need a polymer basecoat which will need to be reinforced and pinned through the Krend into the substrate.