1st perspective: Paralleling wires:
8 gauge wire has .6281 ohms/1K feet.
10 gauge wire has .9987 ohms/1K feet.
Two 10 gauge wires in parallel have .4994 ohms/1K feet.
So two 10 gauge wires in parallel have less resistance and are more efficient than a single 8 gauge wire, and they effective are a 7 gauge wire.
But I don't think that is what is being suggested, and the two #10s are more expensive than the one #8.
2nd perspective: Paralleling relays:
If the single 40A relay and 8 gauge wire don't look good enough, what if you parallel two 30 amp relays? Well just above it's clear that the wiring part is OK. So now were' down to paralleling relays. In this current range, relays use a silver oxide plating that by design pits, remelts, and changes as loads are applied. In fact, you should not use some of these high current relays for very small loads (like less than 200 milli amps) because they need the current to have the contacts "keep their form". So the problem is that the contact resistance of the two relays is never the same, so the load is never evenly split. This is also true if you double up contacts on a double pole relay (where there is one coil). An additional problem is that the contacts don't close at exactly the same time, so for a brief period of time one set of contacts will be above their rating. This will affect the contact life. Same problem on release.
Now once the relays are all closed, then you might get a 50% improvement or so in the total current capacity of the system due to imbalanced current sharing between the different resistance contacts. So your dual 30's will maybe perform like a single 45 amp relay. And you got twice the coil current. If you've got that much coil current, go up to a 60 amp relay (I deal with form factor concerns in a bit). But this is at the expense of contact life/life of the relay. I'd use the 50% improvement for a double pole in the same case, and expect about 1/4 the life. If you're talking two separate relays (like two ice cube relays) I'd expect it to be worse.
If you did this and energized the relays under no or low load, you'd still get the current sharing and not have the contact wear problem. But who here has not got into a car the next morning after driving during a raining night and had the wipers and radio blast on when you crank it. I know I've left the cab fan and AC on full and then turned the tractor on. So this is not an option.
Long story short: Don't parallel relays to get more capacity. In the long run, it will go bad and may do so most dramatically (fire).
3rd perspective: Split circuits:
Now look at two 30 or 40 amp relays, and the correct wire size from them, with each relay and wire driving about half of the circuits in the cab add on. If those circuits can be split up, this works fine and it's an economic or logistic call as to if this is a good idea. Information missing thus far in this post is how and where the wires for the cab connections exist. Is there just one big wire for switched battery to the cab that feeds a bank of switches, or is there a wire per major circuit (lights, fan, etc) or wire per switch? Mention is made of the "switch panel overhead" and that "all fusing must be done independently". But it does not state if there is a wire per fused circuit or switch or just one big feed. The location of these wires is also not clear. If it's out of the bottom of the cab, then a fuse block can be added under the tractor with a fuse per circuit. That also means there is room for a bigger relay (as long as there is a fuse close to the battery for the line feeding the relay).
It's not clear if the motivation for the ice cube relay is a desire to use a free spot in the existing relay and fuse panel of the tractor or not. If that is the desire, then a check should be done to see if the main line from the battery feeding the panel can supply the extra 40 amps of current.
With all this information not known, it's hard to suggest a solution. So Steve I don't understand the "two circuits being more efficient than one" but I don't have enough of a picture of how the add on cab is wired and what the OPs wishes are regarding physical layout. It would also be useful to find out if the hardness in a 4120 is the same as a cab version in the 4000 series, as that would shed light on if the extra current is available from the harness or if the cab should be powered off of a new connection to the battery. I have a gut feeling that the molding for the fuse and relay panel are common in the 4000 series, but that the 4120 might have a different wire harness to cut cost and not have the extra capacity for cab power.
Hope this is clear. I actually have posted a few 1-2 line replies in other threads, hard as that may be for everyone to believe.
Pete
All I really meant was to have two relays (or even more) instead of one to carry all the load,and be able to use smaller wire. If it was mine,and I was able to find a nice little panel (that might be the problem),I would have a relay for each motor and possibly split the lights between two more relays. 
