We do exactly what you're looking at.  2015 was rough on our 9770 with a cracked rim, vertical auger bearing and gears, primary countershaft gearbox, broken lug bolt resulting in a new hub, all in-season.  Trade-in at the time was around $120k, one year old S670s with 2xx separator hours and under were all around $300k.  Found a '12 S670 with 850 separator hours for $180k and bought our first draper head.  Ran one in corn and both in beans.  Had a couple years where the 9770 was down several hours, once with a rear wheel motor, and last year with a persistent spontaneous combustion issue where the other one just kept clipping along.  Don't remember any significant time where both were down at once, and our thought in the beginning was surely one would always be running.  We also knew that newer doesn't always mean more reliable, and, perfect as we are, have had accidents that cost us a day or two.  In the end the extra draper head is all it really cost us, though we added a second corn head last year more as insurance.  We felt at the time, for about the same money two was better than one and the way our insurance is structured the extra machine is irrelevant, as we pay on total machinery value.  I will say, after doing the same thing with sprayers and planters, we're out of shed space again.