Hi,

Its unlikely that you won't have any soil pipe. You would notice it straight away as the water in the toilet pan would be sucked away and you'd get horrible smells coming back up. This is because a vacuum would be formed when flushing that would suck away any 'plug' of water left in the pan.
What is more likely is that you have a hidden Dirgo valve. This is something that is normally installed when you're doing an extension. It's basically a large Automatic Air Vent which allows air to be sucked into the waste pipework when you flush the toilet but doesn't allow smells back out.
These have to be installed on the waste pipework above the toilet or bath overflow level and is normally hidden behind a false wall in the bathroom or close to a drainage point. You'll know when they go wrong because of the smells starting to appear :-)

Hope that helps.
Ben

You probably have a hidden durgo valve(automatic pressure release valve) This valve would only need to be higher than the highest waste water trap ie. Sink
Otherwise you would know all about it with the smell.

Louis.

Edit.
Both toilets may be serviced by the one durgo valve.
This is fine so long as the existing durgo is above the highest waste trap.
I tend to allocate a durgo to each bathroom but i don't think you will have any problems, simply have to replace durgo a bit more regularly.

Louis.