First of all, welcome to RO! Are these your first rabbits or have you had bunnies before?
I could only see two environments being a problem if one was indoor/the other outdoor and there was a big difference in temperature - rabbits (the vast majority anyway) shouldn't have trouble adapting to a routine that involves two different homes, especially if they get used to it from such a young age. Switching environments is definitely preferable to being home alone for a day or two when you're at his house. You could always bring some things from home when you move them - a fleece blanket that smells like them, a favorite toy or two, etc. Plus, you'll be with them at both locations and they'll have each other for comfort. Keeping other aspects of their schedule the same (or close to it) like what time of day you give pellets, veggies, etc. can help too - rabbits are creatures of habit, so they like when life is predictable.
My oldest rabbit, Nala, has been through a few moves in her 6-7 years with me - from San Antonio to Houston (where she also first encountered cats and the concept of mommy having a boyfriend/husband)... after we'd been there a while, she and Gaz went from being tucked away in the 2nd bedroom to being in the living room. A few years later, we moved to an extended stay motel for a few months and then to a new apartment. The rabbits are always a little bit "OMG, it's all different, what is this place??" but settle in pretty quickly after a move. If they can handle that, I'm sure yours can adapt to spending weekends or w/e at your boyfriend's place. If you act like it's no big deal, are calmly reassuring and behave confidently around them then they'll most likely follow your lead.
If you want them partially free-roam at his place, make sure to start with just the x-pen area and gradually offer them more space to explore. Extra litter boxes in various locations can help as well. The biggest issue you're likely to encounter is that if they don't recognize the territory as being familiar, they might leave excessive poops (or even pee out of the litter boxes some) as a way of marking their territory. Since they're switching homes, you'll want to expand territory in each place at a slower pace than usual.
Just so you know, even if they're the same gender they'll need to be spayed/neutered as soon as possible (age/weight requirements vary by gender and vet) and until then, you'll need to keep a close eye out for signs of hormonal behavior. This starts as young as about 12 weeks in females (not sure about males, it might be slightly younger that it can start). If you see circling, humping, find clumps of hair that look like they've been pulled out or any other indications of scuffling then you'll need to separate them until they're fully recovered and no longer hormonal (about 4 weeks post-surgery for females/6-8 weeks for males). Otherwise, rabbit fights can become vicious or even deadly... and even if no one gets hurt, they can hold grudges about the fighting and have trouble bonding as adults.
My first two girls got spayed at 5 mos old (vet wanted them to be about 2 lbs first and they're dwarf breeds) and were able to stay together until their spays... however, they started fighting while in the recovery cage and had to be split up while they were on the mend, then re-bonded a bit.
<3 Lionheads, btw! My heart bunny is Nala, our lionhead. She's amazing and has always been there for me. Whenever I take her out in public, people always really fawn over how unique and adorable she is, hehe.
I don't know if you're familiar with the breed, but there's two categories of lionhead... single-maned and double-maned (depending on whether they have 1 or 2 copies of the gene). They all tend to look similar as babies, but when they blow their juvenile coat at a few months of age, you start to see the differences. Mine's single-maned, so once she lost her baby fur, her mane was significantly thinner/smaller and she lost the "skirt" she had around her hind end for good.
Here's some pics of her (would love to see pics of yours!). The first two are from when she was little, the third is recent):
