It would depend on the specific reason your psychiatrist prescribed both of them as both are FDA approved as mood stabilizers. It would be best to ask them and if they believed that it might work out they could allow you to titrate the dose down gradually to see how things worked out but that would be their decision. You could discuss that concern with them and also use the mood tracker to update them on how things were going if they approved it.
I've had two pdocs suggest lamictal and seroquel for bipolar. I think it is a pretty common mix. I was at my most suicidal taking seroquel so I don't think it worked for me, but in hindsight I also didn't give it a fair try. You don't say how you are doing. If you are doing well don't start tampering with things. It is really tempting to play pharmacist but it doesn't always end well. And whatever you do don't go cold turkey off these meds. It will at the very least give you one nasty headache with nausea and at worst could cause seizures.