It is still stressful but perhaps not as much, but the thing to watch with this situation, is that if they decide to spawn, the divider had better be all the way to the hood or you will perhaps lose them both. If one or the other jumps the divider, one or both could be seriously hurt or killed by the other.
I am using my psychic powers to predict what Rose's answer will be...:
"they may be ok but it depends on the individual fish...have an alternative available in case it doesn't work...and be sure they can't jump the divider"
As with many things, it is going to depend on the fish. I can say that the little ones of Phloxface's divided tank seem to get along fine, but if you get a territorial or aggressive betta on either side of the divider, you can have a problem. If you find that one or the other cannot handle the sight of the other one. You may have to get an opaque divider or somehow fix it so they cannot see each other.
All my tanks in the den are sitting right next to each other and the boys can see each other very well. They went through a couple days of flaring when they saw each other the first time, but I would put a piece of cardboard between them and after the first couple of days all is well. (these are all separate tanks though) I do have to be careful and follow the order that has been set up for feeding though or I have everyone upset. They eat in a certain order and if I pick up the food and feed the wrong one first, there is POUTING like you would not like to see. Bettas, in my opinion, are very jealous fish and do not like to see the other guy get the attention.
Well I was typing this and read the new post by Omorrokh and she is so right, that is just what I said. ;D
I think in almost all cases it works out just fine for a male/female split tank. You should have a backup plan just in case, but you probably won't need it. If you get the Penn Plax divider they do go right to the top but you should get a glass canopy to put under your regular hood (or in place of your hood) otherwise there will be a small space right where the feeding door is where the female can jump through. With the glass canopy the divider fits flush with the divider and there is no space. The canopy is cheap (around $10 or less) and you can just put your light strip right on top of it and not use the plastic hood at all.
When I put the divider in Lava's tank he freaked out for about 20 minutes, not about Karma being there but about the divider. He's a very sensitive, high strung fish and doesn't like change but he did get used to it and enjoyed Karma's company. Lava is now in the 5 gallon Hex by himself and is doing fine. One of my 10 gallon tanks is divided into 3 sections and I have 3 of my girls, Aenara, Angel and Noelle in there. They get along great (as long as they can't bite each other) and interact with each other constantly. My other 10 gallon is divided into two sections with Phlox and Whisper. Phlox is such an easy going fish! He is just fine no matter what I do. He was attached to Karma and when I had to remove her to put her in Whisper's tank he sulked a bit. He got over it pretty fast as Whisper is so active, teasing and playful he couldn't resist her! They play together all the time and are so much fun to watch. Sometimes I catch them kissing through the divider or just racing back and forth together. Phlox was pretty lazy before he got a roomate and not getting enough exercise. Since getting a girlfriend he's constantly active, swimmy and showing off his fins! One thing I've noticed is the females are a LOT more active than the males! Having a female next to a male makes the male more active too.
They are very competitive and jealous at feeding time, all of them! I try to drop the pellets quickly into each of the sections so all can eat at once but most of the time they are so worried about what's going on on the next section they don't see their own pellet. I have to holler at them all the time and point out that they DO have a pellet in their side just like the others! ;D The girls tank is the funniest! All three have different behaviours at feeding time... Aenara is my jumper! As soon as I open the feeding door she starts leaping out of the water. She won't even wait for me to drop the pellet. She is so impatient she jumps up and nips my fingers trying to get it. I try to drop hers in quickly first so she stops jumping. Angel is next to her and she wildy swims back and forth along the glass and doesn't pay attention to the pellet and usually knocks it away with her head as she zooms back and forth. Then there's Noelle, who stands in the corner and furiously wags her tail so hard she churns up the water and causes such a wake she loses her pellets when they float away. ;D Feeding those three takes the longest as I have to get them settled down first so I can get them to pay attention to what's being dropped!
Divided tanks can be a lot of fun! Having the fish together (but protected from fighting each other) really seems to bring out their odd little personalities even more and keeps them active. You just need to be very careful that they cannot jump over and get to each other. You also definitely need a hospital tank as when one gets sick, unless it's contagious, you cannot medicate them in their tank with the other healthy fish.