Oh dear, I think that cutting the cell in half will go right through the roots and hurt them. I saw a video of someone who separated her tomato seedlings with a fork, gently prying the roots apart. The roots in her cells were not too intertwined though.

What kind of seedlings do you have? I've been successful separating my seedlings that I didn't have the heart to pinch off, but I was very, very careful (and lucky too) when I untangled the roots. I tried my best to not pull and rip the roots themselves. My seedlings weren't more than a few leaves old. I think in general plants don't like to be transplanted like that, because it seems like it's too easy to shock them.

I'm still a novice when it comes to gardening, maybe someone else will have more advice. My guess is, the safest thing you can do is probably to transplant the whole thing as it is.

You could try dipping the soil mass in a bowl of water and gently swishing it around until the seedlings can be separated.

I've been separating mine this way and it has worked well. For the peppers and tomatoes, I did them early so that the roots were not too tangled together. I did lettuce but waited too long and those were really twisted all together. I had mostly 3 per cell, some were 2. I used a small metal pick. I think it was for shellfish, I had used it before to pull out lobster meat from inside the legs.

Anyway, I watered well first so the soil would stay together and held the bunch of seedlings in the palm of my hand. I used the pick first at the bottom to loosen the soil and separate the roots at the bottom and worked my way up. I held it so that as I broke up the soil, it would just fall off and the roots just hung down. It was time consuming and I had to be very careful so it is not something to do when you are in a hurry. I did break a few roots off and I lost the first couple seedlings but the rest are doing really well now. The roots did not grow downward so cutting them in half would not work, I don't think. I guess it depends on what you are growing. With the lettuce and tomatoes the roots grew horizontally into the other seedlings. When I did the peppers, I did them small enough so that the roots had not intertwined yet.

I start germinating my tomato and pepper seeds in sandwich baggies in wet paper towels on a tray on top of my aquarium lights.(Too cold in my basement to germinate seeds, I have had very poor results with a heat mat, and have no space upstairs to start seeds in a warm area safe from my cats in a starting tray.)

When they germinate (root and stem with leaves), I transfer them to 72 cell trays ( flimsy Burpee trays work best for me). I put up to four seeds in each cell, one in each corner.

When they grow out a few permanent leaves, I use an iced tea spoon to scoop out the 1/4 of the mix in the cell containing the subject plant, without regard to whether I am cutting through any roots, and move it into a 9 oz. plastic cup of potting mix.
I grow them to a good size for planting out without further manipulation, other than moving them to my greenhouse when I have too many for my indoor lights, then moving them to my outside screened shelter for hardening off.

In five years of growing them this way, I have never killed a plant by damaging roots with my iced tea spoon! Every year, I seem to have one or more "runts" that only seem to have one or two roots, but, so far, they have all gone on to be good looking plants by planting-out time.

The chance I would spend time untangling the roots of a few tomato or pepper seedlings? ZERO!

Oh wow, Tdscpa! That's an interesting method. It sounds like you've developed a successful (and easy) process of growing your seeds. It makes me feel like I probably am making it harder for myself than I need to.