I'm in the process of developing my basement. There is no rough-in for bathrooms. I'm putting an ensuite attached to a room with a shower, vanity and toilet. Backing on to this bathroom is another 1/2 bathroom with toilet and sink. There was no future vent put in when the house was built making venting a huge problem. Any suggestions? i.e. Studor vents or venting horizontally to an outside wall. Next question, do all the drain lines tie into one line that leads to the main drain?

First of all, what's a "ensuit" ? My online dictionary doesn't cover it?
Hey! You're trying to complicate a easy remodel job. Since all you want to install is a toilet and a lavatory and planing on backing this group next to a existing bathroom group, why not use what you already have.
If this were my job I would break cement from the existing toilet under the wall over to the rough-in on the other side. I would remove the existing closet bend,(save it, you will use it later unless it's cast iron and you wish to convert to PVC) and replace it with a upright combination 1/8 bend and wye to pick up the old toilet . I would extend the open end of the tee over to pick up the new toilet using the old closet bend I would feed water to the new toilet by replacing the 1/2" elbo in the wall with a tee and stubbing out on both sides of the wall.
For the lavatory I would open up the wall and replace the existing tee with a drainage cross and stub out on both walls. Do the same with the water as you did on the toilet changing the ells for tees.
The existing lavatory vent will vent both groups and the cement that will have to come up will be minimal. Sound like a plan? Tom

An ensuite is a bathroom attached to a bedroom. Canadian term I guess. There is no bathroom in the basement yet. I have to break the cement and tie the drains for the shower, 2 sinks and 2 toilets into the main line.

I must have missed the part about no existing rough in. Same deal except to establish a tie in point to the main and extend it over to pick up toilet #1 using the same fittings to pick up both toilets. On bath #1 you will tie the lavatorys in downstream from the two toilets with a 4 X2" fitting and the shower in bath#1 with a 2" fitting taking off from the lavatory drain. You will run the lavatory drain up 18" off the floor to the center of a 2" drainage cross where the lavatory drain will then become the vent for both groups. You may then run the vent out the roof or terminate with a Studor Spring Loaded Mechanical Vent if code allows. On bath group #2 proceed as I have advised. Your water pipe will have to take off from a 3/4 main and countinue 3/4" to the second bath where you may reduce to 1/2". Good luck, Tom