A single motor will need a more complex gear chain, probably not much difference, the OEM cost of a motor will be under a fiver, neither here nor there in getting to the retail price.
and how do you get the ends to slip independently with only one motor? Used to listen to that feature most everyday when I worked for Zambia railways with a 1:50 grade out of the station just a block away, usually lost their footing on the level crossing, first the front engine then the rear engine, then when that one came under control open her up to get the momentum back. Memories of the good times, got to have another look through my Garratt page.keith