You already know, excluding the cost issue that my proposed solution is correct. Its how everyone does it and, if I recall correctly, you have given exactly that advise to others. You didn't say what tractor, but if it like your and my MX tractors, its a ISO 7241 B QD for the backhoe loop. One set of those in 1/2" will set you back $75 for the halves on the top cylinder and you would re-use the ones for the BH.
Doing it the correct way:
3/8 ag QDs are $20/set or $40 for all you need for the top link. The backhoe QDs are re-used for the backhoe.
If you mount your valve right, you only need two hoses for the top cylinder. Just mount your ODs directly using NPT nipples (or whatever nipples you need). Then you need just one pair of about 3 ft 3/8 hoses to get to the top cylinder.
Surplus center sells NPT/NPT 3/8 hoses for $10 each.
In fact, if the top cylinder can stay installed with the backhoe attached, you can just skip the QDs and directly plumb the top cylinder.
You would need probably one 1/2" line to extend the fixed end of the backhoe loop to the new valve. That is another $12.
There are a few fitting and such required too. But the cost of the system is low. The spool valve costs the same either way, so the increase is just a couple of hoses and fittings.
If you are doing this for hire, you have wasted more labor time "optimizing" this design than it is worth. If not, of course, labor is moot.
We haven't discussed the tank line, but its moot as it doesn't change regardless of your choice of system.
A single spool 10 GPM valve is $80 at
Surplus center, a two spool is $120. Would it be money well spent to provide a second spool?
Ask your customer if he is willing to spend a tad more for a better system.