Brief Answer:
you'd better avoid it

Detailed Answer:
Hello,

as you've correctly pointed out they're both antihistamines so you're likely to get minimal extra benefit with relatively more side effects. Sometimes we do try either higher doses of antihistamines or combinations of antihistamines for resistant allergies but you've got to be aware of the (potential) consequences. More sleepiness, dizziness, difficulty to urinate, constipation, etc may occur with the combination. The effect cannot be individually predicted, therefore you can only find out by trying...

If you do develop significant side effects then perhaps other options might have to be considered. For example for a persistent allergic reaction, corticosteroids can be used.

I hope you find my comments helpful!
Kind regards!

---