Have you asked your neighbor if they would want a simular garage? I have seen this done several times before where two garages are built between two semies with a party wall in the centre. This would maximise the internal width of your garage as the wall would be straddling the boundary. I'm not sure what kind of wall the party wall would need to be, I have a relative whi lives in a fairly new house and her garage is attached to the neighbors garage and the party wall only looks like a 4 inch wall with pillars but it maybe diffrent having the garages attached to the houses. I suspect the cost would not be that much higher than building one garage as the common wall could be built with breeze blocks and only the ends would need to be brick and they would be mostly door anyway.
Funny you mentiont hat, I was thinking this last night, and I know my neighbour woud like a garage but I don't know if he can afford it? Luckuly it won't make our houses terraced or link detached, as next to my house is the 2-driveways and then a garden (we are built on a corner. It's a bit of strange set up as my semi is one of only two houses joined, the next two semis are around the corner, but one of their driveways is next to mine.
I'll have a word with him this weekend, just wondering if we did this what the 50/50 split would be on a ball park figure?