I was going to suggest Zooplus & Pets at Home but great thet someone else has already trailed the sites in front of you. Looking at reasons for not having two together could soon be put into perspective,if his dad is allergic what difference then one or two,actually a lot of people who are allergic to cats are actually fine with MC's, if one cat can't meet the dog again what difference two,in the first year to eighteen months two babes would be able to travel in the same carrier & if you could get one with wheels {covered in another thread about pet carriers on the forum} you most probaly could put your own baggage on top of that although of course you will find you pack more for the furries than yourself,when we do cat shows & stay away for the night I travel with toothbrush & clean underwear the rest of the bag is took over with their "might needs"....!
The second babe that we had as an extra was actually always with the one we were having,right from the day they were born they would both be side by side when feeding & once they started getting around if you found one then the other would also be there,when we went to collect them they both sat side by side & we always reckoned that at that point if we had still decided on the one only we would have had to give in because no way could we have split them,never regretted the decision though,they are both very different in size & temperments but great fun,eyes in your backside doesn't help when they both gang up against you.
If you are only having the one now though it is great to know that down the line you have a Teddy playmate to look forward too as well & the waiting & fun starts all over again...x