I don't know that they always keep them to 2 stems. I've bought some that had more than 2 stems.

I also don't think they are all exactly the same size, though they same variety will mostly be around the same size--
A-- because they used good seed--not saved seed and were started at the same time and
B-- because they are grown in optimal conditions so they will all grow roughly at the same rate--and
C-- because they cull out the ones that grow differently.

I start a few geraniums from seed every year but the bulk of my geraniums are from cuttings of plants I grow all winter inside. Cuttings produce flowers earlier, but I like to refresh my stock so I do sow a few seeds. I don't open my greenhouse before the middle of April--check my zone. I start the seed in the house in February. I move them to the greenhouse when I open it, and put them in bigger pots. I set them out in the beds at the end of June and they start blooming in July.

My cuttings however are blooming when I set them out

My daughter showed me from her friend who worked in a nursery, works for me.

Root one cutting cut about 1/4 inch below a leaf joint, strip off the leaves on the bottom, set in a clear glass of water in an east window, change water every 5 days or so, stem should be about 3/8 to 1/2 inch thick. I never have to use rooting powder, takes about 3 weeks. Don't worry if a few top leaves get crispy but pinch off any buds.

You can put 3 to 5 cuttings in one glass of water; I like the smaller glass jars marshmallow creme comes in but anything similar will do.

When there are several roots about 1/2 to 1 inch, fill a pot with soil, scoop a depression, gently set the rooted cutting in the soil deep enough so it won't topple over and gently firm soil around it. Avoid air pockets around the roots, water well, empty any excess that drains into a saucer.

After it produces top growth about the same size, cut it off about 1/4 inch below a node, strip bottom leaves, stick your finger down in the soil forming a V, and just set the unrooted cutting in it, firm. It should root.

Can't advise about timing, probably December to be ready to set out in after last frost in the sun. I don't harden them off but don't put in full sun the first week or so.

Have done it many times now. I start them any old time, but to set out in the spring, probably start in December.

You could probably stick two unrooted cuttings in potted soil like that forming a V, haven't tried it but may be faster. You can probably add a 3rd or 4th cutting that way if the pot will be larger. I usually use a one gallon pot.