Code may vary where you live
Yes. You are going to create a "Shared Neutral" circuit. This isn't difficult or dangerous, but you should do some labeling in the panel and the box itself.
First, it looks like the receptacle is a 30A/220V grounded receptacle (the wire itself may also be 30A -typical for an electric dryer- but that doesn't matter since you are using 20A breakers and receptacles). You are going to share the neutral and ground wires for these (2) new circuits by wire-nutting the two circuits in your first location (the junction inside the GFCI).
Shut off power to the 40 amp double pole breaker.
Assuming you have a red, black, white, and green/bare wire:
The red is one hot.
The black is the second hot.
The white is the neutral for both.
The green/bare is the ground for both.
Assuming that the "red" circuit will be the GFCI at this location, connect the Red to the hot\line side of the GFCI, make a pigtailed white wire to do the same. On the load side of the GFCI, run the continuation of the circuit.
Using the Black wire and another pigtailed neutral, junction these to the line that is going to the other room.
To be a good citizen: Label the inside of the box "Shared Neutral". Extra points if you "tie" the two hots together with a piece of white electrical tape with some extra length so it looks like a little "flag" that says "Shared Neutral". Double extra points if you label the cover plate so that someone sees it before they make any future modifications.
Go to your panel. If you are not comfortable working in the panel, shut off the main power and probably have a friend hold 1 or 2 flashlights for you.
Disconnect the (2) hot wires from the 40 amp double pole breaker (I assume these will be red and black). LEAVE the neutral and ground in place.
Remove the breaker and simply replace it with (2) 20 Amp breakers. Keep the same position in the panel OR make sure that the breakers are on OPPOSITE poles -as if they were a single 220 volt circuit. IT IS IMPERATIVE THAT THEY ARE NOT BOTH ON THE SAME POLE OR YOU RUN THE RISK OF SENDING 40 AMPS THOUGH THE NEUTRAL ON THE SAME POLE (If you need further explanation, please ask -this is the most important safety step). Some people will disagree and will say to use a 20 Amp double pole breaker. The problem with this is that it inaccurately appears to be (1) 220 volt circuit instead of (2) 110 volt circuits with a shared neutral. There is an exception to this rule below.*
LABEL The panel cover by bracketing the (2) circuit labels and write "Shared Neutral".
Extra points: inside the panel, tie the (2) hots together (the red & black) with a piece of white electrical tape that says "Shared Neutral".
NOTE: You will NOT be able to split the circuit after the GFCI ...so don't think you can give GFCI protection to both circuits with (1) GFCI receptacle. If you want the other circuit to be GFCI protected, you must use another GFCI receptacle in the first location of the second (black) circuit. You also will not be able to use a single pole GFCI breaker on either circuit. You must either use (2) GFCI receptacles as above. Alternatively, you may use (1) double-pole (220volt) GFCI circuit breaker -but you should still label the circuit as "Shared Neutral".
- EXCEPTION TO THE (2) 20 AMP CIRCUIT BREAKER RULE: If you want these circuits to be AFCI protected you must use (1) double pole (220 volt) AFCI circuit breaker. You should still label the circuits as "Shared Neutral".
...That was fun to write! :D
Peace,
Greg
Masonry boxes are metal because concrete can get wet and conduct electricity. The box can be grounded to prevent this becoming a problem. Code allows you to use items that are MORE safe than is required. For instance, if code requires 14 gage wire, you're OK using 12 gage (but not 16). The same holds here. You're proposing using a more safe box than is required. As long as it physically fits, you will be OK.
Best Answer
As the good commenters mentioned you cannot bury the junction box, it must remain accessible. A hinged bench seat sounds like a great workaround. That space under the bench could be considered part of habitable space though, so the cable would need to be protected in a raceway or conduit. It would be acceptable to build a short raceway out of 2x4s.
But if you're going to have a hinged bench anyway, would it be so bad to just leave the receptacles as-is, and when you need them open the bench? You could leave a little notch in the back of the bench so you could close it after plugging in the vacuum or whatever you'll be plugging in. Or run a power strip out of it.
[EDIT: A cable coming out the blue box and into the void behind the wall could route into a raceway that terminates into the same wall void, adjacent to the blank cover. The reason you cannot have a cable run out of a hole in the blank cover itself is that would be unprotected. Protecting that cable coming out the coverplate makes the junction box inaccessible. What you could do instead is use an extension box that you could screw conduit into:
The box extension comes with some plugs to plug one of the holes, and the other hole would be your plastic or metal threaded conduit terminal adapter (must connected ground to it if its metal).]