Wow! OutSTANDING!

I'm frequently cautioning TBers that a 400W
RMS amp can easily damage a 400W
Program cab. The difference in these measurement methods is too often overlooked.
That section of the manual referring to "multi-amp configuration"
assumes you're using two Rumble 800 HDs, since there is no basis to assume any other configuration
! 
I have 2 Bassman combos that work great like that!
Compare the different
Rumble Block Diagrams for the 100 and the 800 on
your Rumble Club Wiki page. The
Master on a 100 is upstream of the Send/Return jacks, so it
will affect the Send signal but, because of
the Cutoff Switch in the Return jack*, it cannot affect a Returning signal. And the
Master on the 800 HD is downstream of the Send/Return jacks, so it can't affect the Send signal, but it will affect a Returning signal.

Don't worry. As long as it's just a Send signal going to a Return jack it cannot overpower the secondary amp. IIRC, someone posted here that the Rumble 40 thru 500 power amp always runs full on (or flat out, if you wish)
!! If that is true on the Rumble 800 HD also, then be sure to keep the 800's Master way down at first if you connect the 100's Send at max volume to the 800's Return
!!! Not that it could blow anything, just that it could be much louder than you expect, since the 100's Master affects its Send volume.
Please feel free to ask clarifying questions. This can be complicated, and I'm writing this late at night (sleepytime) for me.
*The
Cutoff Switch in the Return jack functions by opening when plugged in so as to block any
internal pre-amp and/or Send signal from going to the power amp and speaker(s) and allowing only the Returning signal to be amplified.
But it also
malfunctions by getting corroded in dusty/humid environments and slowly/slightly opening the switch to allow only an
intermittent signal to be amplified
!! Severe corrosion may not allow
any signal to be amplified
!!! Diagnose this by connecting the Send to the Return on the same amp to see if it fixes the problem. If so, run the plug in-&-out of the Return jack several times to knock off the corrosion. If that only partially fixes the problem, a
small amount of Deoxit on a Q-tip can be used inside the Return jack to further clean the Cutoff Switch.
.