The present system is more than tainted when it comes to working with outboard as inserts, especially when working with pairs of mono devices.
Refer to this article for in depth info on why it sucks:
Here’s a good quote from the author:
"When I shared my concerns about the limitations of External FX routing, Steinberg agreed the situation wasn’t ideal, and said they’d rationalise the External FX system in a future update (probably in Cubase 7). "
quite amusing to see none of this seven years down the line…