Bexmanc,
Depends when you start training. I'm a bit of a shoe whore, and have about 4-6 pairs on the go at any time, including xc ones. (Mrs D is convinced there's an Asics octopus under the bed).
If you're training now, I'd alternate the two all the time, this allows them to dry off between runs. I usually buy a new pair 4 months/16 weeks before the marathon, and use them for 2 runs a week max, so that by marathon time, you know they are 'run in' but not knackered.
In your case, perhaps use 1 pair for the majority of runs, using the second pair for maybe the odd long run, and short recovery, as the marathon approaches, then start to use the 'new' pair more.
Usual guidance is 500 miles for each pair of shoes, with 35 weeks to the marathon (at a guess) thats only about 30 miles per week, so you'll probably need a new pair before then anyway.
(BTW I usually get 6-700 miles out of mine)
Good luck