Seville two days-best way to get tix to cathedral and alcazar

The official sites for each will have tickets.

When people say "skip the line," they're talking about the line to buy tickets. What line are you hoping to skip exactly? In any case, just buy the tickets from the official sites. You're not missing some better version of the ticket.

For the Alcazar, I would buy a timed entrance ticket in advance on the official website. At certain times of the year, it does sell out.
The entrance is Lion Gate (very obvious when you arrive - directly facing the cathedral and painted red with big lion picture on it).
There may be a queue, but this is only for people who have already bought tickets for the same entrance time as you (the ticket office is around the corner). So once your time opens, the queue moves very quickly since they are only checking you have a ticket, not trying to sell one.
Does that make sense?

I agree with what was said above. We were there in May and had our tickets bought online before we left home. You skip the long line of people waiting to buy tickets. You get in line when it is your timed entry, and that line moves quite quickly. When it was our time to enter the cathedral at noon, we just walked up and immediately entered with nobody in line. The Alcazar line moved quickly and if anyone was in that line for a different time slot, they made those people move to another line to wait their turn. It is well organized.

Thanks all-so sounds like I’d buy them separately from the actual websites for each place.

No, you buy tickets from the official website, and then you show up at your pre-booked time with your ticket.

It’s as simple as that.

I thought that was what I said! Just syntax. But thanks for your help. Will book from official sites this weekend.