Is it safe to take 15mg of mirtazapaine with 25mg of zoloft (sertraline)?

Probably: Both drugs can raise serotonin - theoretically to dangerous levels - but that would be very unlikely since 15 mg is a low dose of mirtazepine and 25 mg is a very low dose of Zoloft (sertraline).

Should be OK: If you are no longer taking Citalopram, it should be fine to take the Zoloft (sertraline) 25 and the 15 of mirtazapine together, which I assume are for depression and anxiety. The mirtazapine should be taken at night as it will make you very sleepy, but Zoloft (sertraline) is typically taken in the morning.

Yes: It is a not uncommon combination. Mirtazapine can help with sleep, and sometimes that's important because -- even though Zoloft (sertraline) will help with sleep that is made worse by depression -- Zoloft (sertraline) itself can have some negative impact on sleep by causing some increased vivid dreaming. The combined action of the two antidepressants can also work better than one alone.