What happens when both of the H9's are bypassed? Is the default routing input/output 1? I would like the signal to go straight into the front of the amp with both H9's off.
If you use relay bypass it will connect each input to the corresponding output. So, this will connect the guitar to the amp, but it will also connect the effect loop input to the effect loop output. You probably don't want this, but have no choice when the unit is off.
You will usually use DSP Bypass when the unit is on. There is no real concept of a default routing – it depends on whether you are in PRE or POST mode, and whether Killdry is on.
The problem here is that PRE/POST when used with a parallel effects loop is difficult when you use more than one effect unit – this is not our fault, it's the nature of the beast.
Here follows the UM definition of PRE/POST operations:
PRE – a mono signal sent to input 1 is routed through the effect to output 1, while input 2 is passed unchanged to output 2.
If KillDry is set, output 2 will be muted. When Bypassed, output 1 will always be dry.
POST – a mono signal sent to input 2 is routed through the effect to output 2, while input 1 is passed unchanged to output 1.
When Bypassed, if KillDry is set, output 2 will be muted. Output 1 will always be dry.
Your problem here is that if KillDry is set on either unit, if either unit is in PRE or bypassed, the effects loop send (output 2) will be lost.
If KillDry is not set, it will do what you want, except that if both units are bypassed, you will get feedback around the effects loop – not good.
So, your other questions:
If I engage both H9's, pre sends the signal to the front of the amp and post sends it to the fx loop?
Yes, if KillDry is off.
If I engage only the first H9, does it follow the same path as above? what if I only engage H9 #2?
See the above discussion – depends on KillDry.
Sorry – as I mentioned earlier, this gets difficult with more than one effect.