It might not be easier to permanently bond now. It may actually end up being more difficult. Both need to be spay/neutered in order to be together regardless of sex. If you do decide to go ahead and get one of the littermates of the same sex, be prepared that in a month or so they may need to be housed separately until both are altered (boys at 3.5-4 months, girls at 6 months). In most cases, your best bet for a permanent bond is opposite sexes, so they will need to be housed separately from this point forward. Since they will not be together during this time, they will basically be strangers anyways when the time comes to bond them together.
Have you priced out how much it will cost to alter them at some local vets? I would strongly advise you to look into that before making a final decision on getting another baby. What I recommend is once your bun is older and neutered, you take him to a local rescue to do bunny dates and adopt a rabbit that is already spayed. This way, included in your adoption fee of 50 or 75 dollars is the cost of spaying. Also, you are able to save a rabbit and that opens up another rescue spot for a rabbit in need.