Don't beat yourself up about this Wendy. As you have identified, you have two 11 months old boys. Two "Kevins"
. They could have easily got on together - although I think to be fair the chances of it not going so great were probably higher
.
Hindsight is a wonderful thing and there were things you maybe could have tried to ease the situation before bringing Harvey to your home for this visit, but even that would not guarantee they would get on.
I think Harvey's behaviour is fairly typical for a pup in his situation. He will have felt very threatened and responded the only way he knows how.
I'm sure all will be happier now he's somwhere else.
The worse thing you could have done would have been to try and force them to stay together, especially as this was only ever a temporary thing anyway.
btw - imho I think the fact they are brothers is irrelevant. It's the age and gender thing that's the bigger cause of this.