Thanks for the congrats and no probs it is appropriate question
or at least one I can answer
Nothing to say you couldn't take a Piriton when on mirtazapine but like you say they are both very sedative so may make you more drowsy/heavy than one or the other alone. Mirtazapine works in exactly the same way as Piriton does to create the sedative effect by blocking histamine receptors (it works differently to create the antidepressant effect).
If you did want to take both then try taking them at different times of the day to try to reduce the sedative effect. Alternatively take a non-sedating anti-histamine such as loratidine or cetirizine (assuming that they work for you as I know some people find that they respond better to certain anti-histamines over others)
Just want to check if you are still breast-feeding the twins or not? As this would affect choice of drug.
Hope this helps and isn't too late a reply
Hope the mirtazapine helps too.
Lots of love