I think the main issue for the twins will arise when they want to know who their mother is. Is it the egg donor, or is it the surrogate, or both? This is also an issue for anyone who uses a sperm donor or donated eggs because of fertility issues or perhaps because they are two women in a lesbian relationship. It is not an inconsequential issue.
As for Ollie and Gareth they are both good people with a lot of love to give. I'm sure the children will be as loved and chrished as any others, and perhaps even more so than some.