I have bred and shown Rottweilers in the past and still own a Rottweiler and a Boxer along with my Cavaliers.
It is not advisable to bring up two pups together unless you have a lot of time on your hand to spend with the pups and in training them. A Rottweiler will need to be socialised and trained. You cannot keep it locked away all day and just take it out for a walk twice a day.
Rottweilers can also be quite demanding and need their training to be reinforced at different times in their life.
At the same time more importantly you need to decide on the sex of the pups. Generally if you have 2 males they will be fine but if they do get into a fight then after that they generally will always be enemies unless you have Ceasar come help you. This was the case with my boxer, dont know abt labs they are a bit milder, but issues can occur and you need to keep the future in mind. Having two big dogs and keeping them apart alway can be quite a job.
My personal advise will be to enjoy one breed first and give it all the attention and then later on after a yr or so think abt another one.