As many of you know, our two boys got on fine for 3 years, then had some nasty fights and were kept mostly separate for 4 years, and then after moving to our new house they seemed to have forgotten it all. I still separate them when I go out, but am confident to walk them together, to have them snoozing round the house together, to prepare food for us or them with them all around me hoping to be fed, and so on - just like a normal pack! However we have 2 holidays coming up where as usual, I am sending the boys to different friends, as it's not fair to expect them to have that possible problem. Is there anything in particular I should or shouldn't do to make sure this happy state continues when we get back after a week of the boys being apart again?

i would introduce them on neutral ground ie on a walk and then bring them back to the house if their ok. i would also make sure they know your in charge by not giving affection to either dog straight away until their both in a calm mood and then only on your terms.
maybe worth having a stair gate up at home just in case you need to keep them apart for a while well they get used to each other again.
You may find you have know problems at all but i'd always be on your guard if they have been fighting in the past.
try and burn some energy off them as well, so their less likely to be full of unwanted energy around each other.     

We have a stairgate up at all times anyway, so that will be ok. I was thinking that neutral ground as if they were new dogs meeting might be the way to go. It will probably be only me picking them up in June, I'll see if I can persuade one of the dogsitters to pop to the park with me first, otherwise it will be fingers crossed! Oh yes, and the dogsitters are a 90 minute drive from home, so I can either do the park near the dogsitter then drive home (with them in separate crates next to each other) or drive home and then go to the field near our house.

i would do the walking before you put them in the car, if you can. like you said then maybe one of the dog sitters can help you out.
hope it goes ok.