We've always split Xmas Day too, and it's worked very well.
One parent has ds2 all of Christmas Eve day until Christmas Day morning- around midday/one o'clock. The parent who has that 'shift' takes ds2 to the other home for that time. Christmas dinner happens after that- around 2pm.
Then the other parent has him from then until Boxing Day evening.
All members of both of our families have worked around this- so last year, for instance, as it was my turn to have DS2 on Christmas Eve, we had Christmas dinner then. When XH has him, they do the same, so ds2 gets two Christmas dinners.
When he was younger (he's 15 now) XH used to bring him over (or he would come over) and we'd all do present opening together. Now he's older he prefers to have two separate present opening sessions.
We offered him the choice of alternating Christmas Days, but he didn't want to not see either of us on the day itself, and prefers this arrangement.
As your step-children are 8 and 12, you should be asking them what they would prefer, I think. But I would also ask that they come to you earlier on Christmas Day, and that Christmas dinner is with the person they are with from Christmas morning onwards. And certainly it shouldn't be any big deal to ask your family to accommodate the two young members of your new family.