I think it's a good looking layout, just a few practicalities to consider I think.
Is the central yellow area going to be cut out? It seems a bit small for access if so (5.5' by 2.5'?).
When you say the outer loop is raised do you mean it uses a viaduct to go over the stream top left? If so it should be okay but it depends how high the viaduct will be. Inclines should stay below 3% if possible and no more than 2% ideally.
If the viaduct in your picture is just the black corner then you appear to have 12' available for an incline (sidings have to be on level ground as should curves). Divided by 2 (what goes up must come down

) that gives a maximum viaduct height of 2.16" at 3%. Unfortunately that assumes a 50/50 split and the sidings at the bottom don't appear to allow that. What you might be able to do is have the bottom sidings be on a mezzanine. If you then moved the solitary turnout further right that might allow you to drop the track down to ground level and thus retain joined loops.
Red lines indicate inclines.
I have joined loops and they are a great idea. I can run a train around my outer loop, another on the inner loop or just a single train running both loops in turn. In your case that's not so easy as you only appear to have one way access (ie;to get on/off you'd have to reverse down the incline I've modified). You
might be able to use a diamond crossing half way along the track I've changed (on its own little mezzanine) so that the descending incline crosses the ascending incline. That sounds a bit funky but I take the view that as long as trains don't derail you can lay track anywhere and any how you like
If you want a taller viaduct you could go all modern and cut out that corner of the baseboard and have a cliff between the curves of the inner and outer loop. Basically you'd build a gorge top left. That would be fun to build
