It can take some time for the current bun to adjust to the new bun. I would not put the cages next to each other right away. You should quarantine the new one for a few weeks anyway to make sure she is healthy. When you are ready, you can put the cages in the same room so they each have their own space. If they get along, you can move the cages next to each other.
You may need to think about their diets in the long term. Even though you will probably wait until she is spayed until bonding them, they will have a few different needs. Portion control could be tricky and if the new one needed different pellets it could be hard to make sure they don't eat each others food.
With bonding, the breed, size and even sex don't really matter it is more about the personalty of the rabbits. You can have a Netherland Dwarf bond with a Flemish Giant and have 2 males or 2 females bond. On the same note, 2 rabbits of the same breed and of opposite sex might not get along.
2 pounds seems small for a full grown Holland Lop. I believe they should not be bigger than 4 pounds, but 2 is small. I have a full grown Lionhead who is 3.6 pounds and is about the size of my friends Holland Lop.