I thought it was just additional ventilation required, with adventitious ventilation knocked off for 1 fire only? Also providing that the flue flow was adequate and one wasn’t counteracting the other?
Do they share the same flue?
Multi appliance ventilation, yes maybe you're right but it depends, like a lot of things in this job lol.
If a wall has been removed between two rooms leaving the two original fires, so long as they are similar/same type with similar height flues and each is less than 7Kw, it doesn't necessarily need additional ventilation. You wouldn't require ventilation if the wall remained so the general theory is that there was enough adventitious ventilation before the wall went, so there must still be enough now, if not more.
There are a good number of things involved in this of course which may lead to it being a plain 'No'.
This case is the other way around isn't it? we don't yet know if this room was so big that it had two fires installed originally and they split it or if the room was big enough to split and then they installed two fires or an additional fire to an existing one.
That's why more detail is needed i.e. room size, fire type, inputs, flue heights, how the scenario came about and what was in originally etc.
Having not seen it personally I cannot say what I would think if I were stood there in the room. It may need multi appliance ventilation calculations as you rightly pointed out or may need nothing. It was more the 'you can't have 2 fires in one room', remark that sounded a bit cut and dried to me. The first thing that went through my mind were all the instances I have seen.
The service Engineer may be well experienced and have seen something that makes it a definite no.
They could also be lacking in experience with these situations as you see less and less of them.
