Other than I think you're nuts?
I guess it depends on your current dog's temperament. Is he rough? Playful? Quiet? Does he love dogs? Puppies?
I find that the typical 1.5 year old male Lab is still a little rough and you have to protect the baby. Even as that baby grows, because it's a Lab, it's not likely to correct your boy for ramming into him like another breed might. I would say that you need to intervene BEFORE things get crazy. Once things are already crazy, it's often too late and then things can get nuts, too rough, the puppy can get scared, etc. So, find a way to do that now. Whether it's having the big dog drag a leash and then grabbing it when things look like they might get to be too much, clapping or even saying something like, "HEY! LET'S GO GET SOME COOKIES!" which works for my boys when things are about to get over the top. It just stops them in their tracks and allows them to settle. I do it mostly when we are playing with other people's dogs because most of my friends do not have Labs or particularly physical dogs and are understandably concerned about my big, physical Lab boys hurting their dogs on accident. I understand their concern because I have seen them body-check smaller, less physical dogs (like 40-60 lbs) and those dogs go rolling.
Find a way to keep them separated because you'll have to protect the baby unless you are supervising. Baby gates or x-pen panels work well. Also, if your big dog doesn't like puppies, you'll have to separate more. I am not looking forward to the day that I bring a puppy home because Linus does not like them. He likes older puppies and young dogs just fine, but he thinks puppies are from the devil. I won't make him love the puppy and I won't put either of them in a situation where the puppy gets to be all over him and I make him stay or tolerate it. My puppy, my problem! Linus will always have an out. He didn't ask for a puppy. It's going to be a rough four to six months for me.
Feed one of them in a crate so the big dog is not taking puppy's food and vice versa.
Do things with them separately.
I would start saving some money, too, because they are so close in age that you're going to have a lot of the same issues at the same time to pay for over the next 15ish years.