You can only guarantee fully shared caches if your threads are being scheduled on the same core (i.e. different hyperthreads) in which case your approach is correct.
But keep in mind that scheduling two tasks on the same core will not necessarily make them run faster then if you schedule them on different cores. L3 which is commonly shared among all cores is very fast.
You need to check how caches are shared among your processors. Most Intel processors share L2 among 2-4 cores and L3 among all cores while most AMD models only share L3.