I've got some similar trackage on my current layout and I'm intending spreading Woodland Scenics Foam Putty between parallel Unitrack, then copydexing a layer of ballast on top of that, so it doesn't seem like such a dip between the tracks where, I think, it would be just be ballast all the way up. I find the Foam Putty is lovely to work with, light weight and finger-friendly.
Looking at track in the UK, the ballast between tracks can peak up.
What I don't want to do is use more ballast than I would have using Peco track!