[jamn]

I don't think so,

The task you are splitting has to be sizeable enough that the setup costs of parallelization (in this case, going to every cashier to put your stuff and then going and picking it up) don't outweigh the potential benefits from parallelization.In this case, unless you are buying the whole store, it just doesn't make sense to bother splitting your shopping cart like this.

Further, if all the cashiers are busy at all the times being useful (ringing customers), you cannot possibly get a gain from splitting any further. There's no way you could improve the throughput of this system when every processing unit (cashier) is at the top of its utilization.