I think its a worry whatever age you bring a new puppy in and theres no guarantee that all will be plain sailing. I've heard girls are worse in fighting i.e. they fight to the death whereas boys have squabbles but as I have no experience of that I'm not sure how true it is. Not every situation is the same, some people rehome their dogs when their is an age gap and its just a bad personality clash and nothing is ever guaranteed.
I wish you all the luck, I'm not sure I could cope with two puppies mind, ones bad enough
Look forward to seeing piccies of your two.