You may choose to keep different varieties of mushroom logs separated for log management purposes, but it’s not critical.  If your logs are physically touching, eventually fungal mycelium may grow out of one log into another. Traditional Shiitake cultivation in Japan was done by simply stacking freshly cut logs touching colonized Shiitake logs! With that in mind, if you stack your freshly inoculated logs touching mature mushroom logs of another species, you might end up with two different mushroom varieties inhabiting and fruiting from the same log. Not what you intended, but kind of cool. Multiple fungal species growing from one fallen tree happens regularly in nature. If logs are not touching, there is little risk of cross-contamination. Spores (mushroom “seeds”) travel long distances by air currents and will land on your log from your nearby mushroom crops and distant wild fungus. However, your planted mushroom species has a strong competitive advantage and there is little chance that other mushroom species will take over from spores. Keep in mind that the logs you inoculated may have already been infected with a wild mushroom species, growing unseen inside. This wild fungus may coexist sharing the resources of the log, or one may outcompete the other. Since some wild mushrooms are poisonous, it’s important to accurately identify the mushrooms you harvest as the edible species.