Do they cremate multiple bodies at once? No they don’t. In the UK the Code insists that each cremation is carried out separately.
Furthermore the aperture (opening/door) of cremator in which the coffins are place is only big enough to accept one coffin.
There are certain exceptions, specifically in certain where the cases of mother and child or small twin children. However the next of kin or the executor of the will need to specifically make this request. In these case both bodies are cremated within the same coffin.
This frequently asked question highlights the opaqueness of funeral practises coupled with society general reticence to confront this issue, plus a pinch of conspiracy thrown in for good measure. You would be utterly amazed by the amount of people who have a strong and vocal opinion of the poor operational procedures of crematoria without the slightest shred of evidence or personal experience. To overcome this many crematoria if requested will allow people to witness a cremation.