Alright, I am answering from memory of things I have read without quoting specific sources, but this may give you things to search if you want to verify.
Until the partial reforms of 1832 most boroughs and counties elected two members but there were exceptions e.g. in Wales where they only elected one, because of the lower population. The City of London (and I think the largest County, Yorkshire) elected 4. After 1832 some 3 member constituencies were created, it seeming more logical to people at the time, if a county was under-represented, to keep it as one constituency but give it an extra member rather than divide it into separate constituencies as we would now.
I do not know what happened in 3 or 4 member constituencies but in 2 member constituencies electors had 2 votes, but were not obliged to cast both of them. To vote for one candidate only was called a 'plumper'.
There was no requirement for voters to be literate and voting was by voicing one's choice in public at the hustings in the town square or county town, so voters had to be aware that their customers, landlord, neighbours etc. would know how they voted. This was especially true as in most counties the local elite was quite small and in smaller towns most people would know most other people.
Consequently there was some social pressure not to cause local rifts and bad feeling by contesting the election without good reason. Many constituency elections were uncontested. Either the 2 leading factions/ families would compromise that they would nominate one member each, or there was a sufficiently dominant faction or land-owner that it was pointless or unwise to stand against them.
The two member thing goes back to the first Parliament. I do not know if any record survives as to why it was preferred originally, but people then might have found it hard to understand the modern attitude that a single person may be considered adequately able to speak for a whole constituency.
Most but not all constituencies were converted to single member constituencies under Gladstone in 1885, but as another person has said a few remained until 1945.