I will have one Multiplus closer to the negative busbar, and the other closer to the positive switches. Is it okay if the sum of each set of cables (positive plus negative) is identical, even if the individual cables are not? In other words, Multiplus 1 has a 50cm negative and a 100cm positive, and Multiplus 2 has a 100cm negative and a 50cm positive. I believe this is okay since the total resistance will be the same.
Best to use cables that are as big as you can feasibly fit and as close to identical to each other as possible to extract the best performance out of the system. If there are any cable voltage loss differences then you will reduce the capacity of the whole system as it will only perform as well as the lowest performing unit.
Best to position the Neg buss bar beneath or between the multis if possible. Or many use a heavy copper strip to do the job and then take it across to the batteries along the bulkhead or wall. Can look very neat and tidy.
Wiring Unlimited has quite a bit to say on this.
As a side note, oversizing the AC cables has the opposite effect.
