I experimented a lot with doubling and adding similar articulations, layering DS, Chamber, Orchestral and Appassionata strings. My personal results so far:
Sample world reacts very different from real strings when layering. Adding a smaller section to a bigger section doesn't mix soundwise to an even bigger section. The opposite happens: it makes the bigger section smaller.
Same effect with solo strings. If I want a smaller section I add the solo strings on top.
Very small sections with DS sound weak. Adding solo strings adds live and power. Great for chamber.
I made a 10/8/6/6/4-section using similar articulations (legato vib, legato expr etc). Works fine.
I used the same section and added Orchestral and/or Appassionata. Doesn't sound bigger. Doesn't sound better.
I made an 8/8/6/6/4 section using the transpose trick for the 2nd Vls. Works fine.
Using the same samples twice doesn't make a big problem with phasing when the section is big enough. But it never makes the section bigger. It just uses more cpu. Not effective at all. MIR doesn't help there.
What really makes a section bigger is detuning. The humanize function is key to a bigger sound. Even when a single line really sounds out of tune it adds to a beautiful sound in musical context. My humanize faders are always above 60% for detuning and even more for timing. It even works with LASS: the 3 sections layered and detuned to +/-4 makes everything lush.
Adding a roomy section (from other developers) to the dry VSL strings sounds better than all reverbs (including MIR).
My advice: listen to the resulting sound but never count the sample voices. Sample voices are not live players.
All said is my personal experience and my personal taste.