We have DS1 (age 9) and DS2 (age 7) and we are really scrupulous about not favouring one over another. We make great efforts to recognise their individual talents and they get exactly equal treatment and resources. We buy new clothes for each child rather than hand me downs except things like wellies.
However, despite all that, I DO think children 'choose' a favourite parent especially when it comes to getting comfort or extracting favours. DS1 is very close to DW and DS2 is very close to me. We always say DS1 is the boy of the family, high energy, intellectually intelligent but has no idea how to deal with people at all, a bit aspergers spectrum, intense, and sport mad. Meanwhile DS2 can read people like a book, knows exactly how to extract what he wants with a wink and smile, is a complete devil with women, cries off at the slightest opportunity if he cannot instantly win at sport, artistic, quite girly in his interests, needs to be pushed to do anything intellectual.
When they were very young it was hard to handle them but as the two of them have got older they have become best friends and play well together. They do not argue much. We absolutely will not tolerate fighting at all. They actually cry and get upset if separated now. They really are good copanions and compliment each other - they are a killer combination of skills and aptitudes.
The only thing I really wish they would stop doing and which is a great concern right now is that they 'compete' with each other - especially DS2 who is intensely and jealously aware of what his brother achieves. He does this even though he achieves very well and in fact better tan his older brother did at the same age in many areas. Meanwhile DS1 tries to boss DS2 around which DS2 finds very annoying.
In short, DD2 wil always favour you and DD1 wil always favour DH. However, be aware of it and treat them eqally and make sure they are best friends with each other not competing enemies. That hapened to my mother and her sister and they still hate each other 60 years later.