You could connect to opposite corners of the combined pack; take the positive lead to the motor controller from the front pack, and the negative lead from the back pack, or the other way around. Two cells or sub-batteries can be made to share current perfectly, in theory. But that means an extra cable that could be a nuisance.
The other thing is that nearly every battery chemistry other than LiFePO₄ will share SOC pretty well. If the front gets lower in State Of Charge (SOC) than the back, then its voltage will be lower than the pack at the back, so current will flow from the back to the front to even out the SOC. This happens continuously while you drive or while stopped (edit: with the contactors on, e.g. stopped at lights), though it could take some time to even up the SOCs since the balancing currents tend to be low. That effect should prevent balancing problems, but I think it should only be relied on to "mop up" minor imbalances resulting from differing cable lengths and differing resistance of connections.
I think it's worth doing the corner connection thing, just so that the front pack doesn't do most of the heavy discharge work (assuming front wheel drive), with the back pack lazily charging the front pack and taking only a modest share of the motor load. The front pack would wear out faster than the back in that situation.
In theory, the paralleling connections from front to back will only take half the load current, so they could be thinner than the connections to the motor controller. I would not make them half the cross section however; perhaps 1 or 2 AWG higher than the cables to the motor controller. That mitigates a little the space and money cost for the extra full-area cable.
