They're quite different.
The Mash-All covers all the Marshall ground you could ever ask for, from plexi to modern. But the Douplex will do Plexi tones that the Mash-All just can't. It's forte is Plexi, and that's what it's there for.
It's the same as my JVM. It covers insane amounts of Marshall territory, but if I really wanted a Plexi, I'd go for a 1959 SL or 1987x. They will do tones my JVM just can't reach.
If i were to go MTS Plexi, I'd go Rhoadie. It's the same as the Douplex, but with an extra gain stage or something. It gets way heavier than you'd imagine.