I need to add another bathroom outside the bungalow, next to the existing bathroom. The existing bog (only) is piped 3m, via a 4" soil pipe, directly to a septic tank. 

Access to all pipework is easy as the bungalow is raised up 80cm above ground level.

 

Can I just add the new soil pipe from the new bog to this existing pipe via a sweeping 90 degree "Y", or do I also need to add vents somewhere to avoid suction/ syphon problems?

 

 

Coming off on two "y" junctions is perfect if you can configure that, but if you have  one wc very close and another much further away you are going to need to look closely at the the falls.

rule of thumb a 4" drain should fall at 1 in 40 (a 6"  1 in 60)

 

the thing about septics is you cannot change the inlet/outfall they are fixed and explains why you sometimes see a bog built up on a silly concrete plinth.

 

For the vent, well a septic normally comes with a narrow 40mm pipe as a vent but from my experience after only a few months my pipe was completely clogged up with dead mosquito bodies!!

 

Extending your soil pipe up above any adjacent windows will ensure there is no vacuum created and you shouldn't have  problems flushing either toilet. its only a few baht extra and will save  you a lot of worry in future.

19 hours ago, Arjen said:

Just add the new tube. The change that both toilets are flushed simultaneously is very small. And if that does happens ones ore twice there is still no problem.

 

...but...if there is already a problem with lack of vent/vacuum then potentially when you add a second wc the first wc could very easily end up with back siphon or worse still.....an elephants trunk as a surprise!.......i dont want to go there!!