Ground squirrels are colonial, so I have no doubt that there are more than two there. There is no one-and-done method to get rid of them permanently.
As long as habitat, especially a food source, exists, you will have ground squirrels seeking to live on that plot. I don't know how large the area is from which you would like to exclude the ground squirrels. On relatively small areas, snap-trapping can be very effective but it must be linked with monitoring so you can intensify your efforts as new animals emerge. See page B-157 of this publication http://pcwd.info/wp-content/uploads/2016/12/1994BeldingCARockGroundSquirrels.pdf
for pointers on specific types of traps to use. Please note that the linked publication has NOT been updated to reflect current laws/registration of toxicant baits allowed in Oregon. Other common options to use alone or in combination include habitat modification via "ripping" or cultivation, flood irrigation, and toxins (gas or ingested grain with poison). In very small spaces or for protection of high-value beds or plants, exclusion with below- and above-ground fencing can
be effective - but because of the labor and disturbance required, that is definitely a finer-scale treatment. Finally, another option to consider is whether the yard area in question needs to be vegetated at all -- As food availability (plants) declines, habitat quality for the ground squirrels likewise declines along with the carrying capacity (how many squirrels can make a living in a given area.)
Regardless of which method(s) you choose, you need to be aware of several California ground squirrel facts of life that affect our ability to reduce their numbers. First, they change diets, which changes our ability to use certain toxic baits. When green plants are available, they're primarily herbivorous, but once green growth begins to age towards drying out, they switch to eating seeds and other plant structures, including nuts, fruits, and potentially underground plant structures like bulbs. Next, they (adults) hibernate - During hibernation, they go deep into burrows and then plug themselves into safe little chambers. Essentially they become immune to anything we might do to them. Young of this year are not fat enough (energy reserves) to hibernate, so they can still be trapped, but that can be a relatively small percentage of the population. The beginning of hibernation varies by year, but is generally triggered by the first wave of low temperatures AND increased scarcity of food.