It's doable, but you have some tough decisions to make. We are not all that in love with the sound of paralleled triodes, so you may actually end up with better results from just having two power transformers and one output tube. If you build the custom chassis with these questions in mind, you could try out a lot of different designs and pick the one that works best for you.
The shunt regulated Crack is something we have built, but it's more complicated than 2 Cracks in one box, more like a BeePre with two Speedballs.
-PB