Soil stack venting - air admittance valve

I currently have 2 toilets, 1 down and 1 upstairs. They are connected together into a common drainage system downstairs. The drain pipe run for downstairs is internal until it is above the upstairs toilet then it goes through the wall to vent to outside. There are no other vents in the run.
Can I remove the outside pipe altogether and fit an internal air admittance valve instead.

Thanks for looking...

An automatic air admittance valve (AAAV) provides ventilation for negative pressure. A foul drain system must also have ventilation for positive pressure (an open vent) so to fit an AAAV, there must be at least one other open vent on the system; this is also a Building Regulations requirement.

You say a “common drainage system”, is this shared by other properties connected to the same run? If so there is a possibility you could fit an AAAV if the drain can positively vent elsewhere; you need to check.

I have 2 toilets connected to a common plastic inspection chamber downstairs, I believe the bath and sink also connect to this, The outflow from this got to a shared drainage manhole outside that has 4 houses connected to it including mine.... The other houses have vents.
Is it enough that someone else has a vent on this system or must I have a positive pressure vent on my bit.
TIA

I'd like to know this as well.