I agree with others that this is a very difficult age at the best of times and really, holidays are just childcare in a different setting. We did very few holidays when our DC were that age as we just didn't have the money or the energy. But a few tips for when we did, and for when they get a little older:
Definitely divide and conquer. I know you think holiday should be you and your Dh getting time together, but you need to mostly abandon that thinking for a bit. We survived by taking turns to get a break - I'd be lying on the beach with a book while he took DC swimming. I'd take the DC out while he was taking a little walk and a wander by himself.
Also, assuming money is tight, do fewer/shorter holidays but ramp up the convenience. I agree with a PP re Centreparcs. We've done Butlins, Away Resorts and CentreParc and while they were all nice, from a comfort and convenience perspective, CP was by far the best and worth the extra cash. When we went to EuroDisney a few years ago, we chose one of the nicer hotels with a big indoor and outdoor pool on the basis that we knew that would be a huge benefit for us, and it was. If one child is always going to sleep with you, try for an extra room or, at the very least, king size beds. We have long accepted that holidays are usually me and DD in one room, DH and DS in the other....
I can't comment on the allergies but I do think you should make an effort to look at international travel when they're older. Worst case, they have to eat fairly basic food in restaurants and you probably should self cater mostly, but it is doable. And a lovely villa, with a pool, can make up for a LOT of inconvenience. Also, I'm not sure how you self cater on holiday but while Ilove cooking etc and we prefer self catering, I have a strict, "keep it simple, stupid" while on holiday - I'm not cooking complex meals in three parts. It's BBQs, and hotdogs and pizzas and pasta sauces.