@amyef2000 covered it well, you need to keep them separated until spay and neuter. Babies (which they are both definitely babies) do get along but do not truly bond, and the fact that you are seeing some mounting means that hormones are coming in. Both to avoid accidental pregnancies and injuries to your young Mochi.
Keeping them as neighbors means they will still enjoy each other’s company and will not forget each other, so don’t feel bad about that.
Boys can be neutered as early as their testicles drop (usually between 3-6 months), and females can be spayed at 4-6 months, depending on how large she is and what your vet recommends. After the procedures, you’ll want to wait for hormones to settle for a few weeks.
In the meantime, I really like using NIC cube grids and x-pen fencing to create temporary housing, or to divide up rooms for free-roam time. You’ll want to have the fence have a buffer of 6 inches to prevent nipping through the bars, which can happen suddenly. They can also mate through the bars…so having the buffer is important! Perhaps Mochi can have access to one room, and Boba can have the rest of the house. Or you could pen Boba part of the time and give them shifts of free roam time (like one gets the morning and other one gets the evening).
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.