Is there a way to share two internet connections simultaneously?
No - this will be hard on two levels.


Your default routes and network routing would be quite involved to set up the correct paths for the Mac OS to route traffic along the paths you defined as opposed to the route each network would prefer.
The graphical interface has no design / affordance to set up more than one route (currently)


The closest way I could see to accomplish this would be to run a second Mac OS virtually and move C and D to the virtual machine with networking set up in bridge mode. That way you would be close to just having a pair of OS with a pair of forwards in place - one per OS.