Harrogate mum, I sympathise - mine wake each other even in separate rooms. I slept them apart around two weeks old because of this and its still happening but certainly not so often. DT2 is generally a good sleeper except if there's really a prob (illness or teething). DT1 is a dodgy sleeper (sadly like her big sis who's nearly 5 and still gets up for a wee in the night).
I let DT1 scream it out for quite a few weeks once I realised there was absolutely nothing wrong! DT2 eventually stopped waking and now tends not to cry in the night if DT1 does. BUT I did have quite a few nights where both of them cried and DD1 woke too and she and I had to do our best to ignore it and get back to sleep (and my DP works away and it was soooo lonely). It was really hard but I reasoned that if I didn't do it then I'd have to do it someday.
Look at everything first, enough day time sleep, daytime sleep coming at the right time (mine are put off their routine if the day nap comes an hour too late), have you changed ANYTHING even in a small way that has confused their little bodies?
As a sleep deprived I really know where you're coming from and wish you all the best.
Advice for christmas - do it to the best you can and ignore any comments that say otherwise (others are not walking the same road you are). Get them to help as much as possible and sometimes this means the hard stuff i.e. providing an ever changing need to keep babies occupied giving you peace to prepare and cook. Sometimes they say they'll cook and pour drinks but sometimes YOU want to do that bit - choose the bit you want and tell them what you want them to do. In my house I tell everyone I'm crap at remembering glasses need filled after the first time and to feel free to help themselves.
AND lastly, if you get bolshy, remember its your Christmas too and take a step back and be a guest in your own house. You are merely providing the venue. Hopefully they'll have a shit time and next year you can wait till the DTs are sleeping and stuff your face with 2 M&S turkey dinners for one!!
Sorry if this is a bit cynical, don't mean to be harsh! Best of luck honey x.