Rather than contemplate whether to invest in two bikes, I would challenge the idea of purchasing a bike like the Domaine at all.
Given your comments about your health and the likely type of riding you'll be doing, a bike like the Domaine seems ill suited to your needs. Lightweight, attractive carbon bikes are natural to lust after, but practically speaking, they aren't that useful to many folks. Do you want to ride VERY fast in group rides? Do you want to race competitively? Do you want to try and beat your friend's time on local climbs on Strava? Those are the things that carbon racing bikes are fun/useful for.
My suggestion would be to buy one bike: a "light tourer." There are various terms . . . credit card tourers, randonee bikes, etc. Basically a reasonably light, but sturdy road bike with some practical design considerations. Examples would be an older Salsa Casseroll, a Soma Smoothie ES, etc. The big benefit versus a true road bike like the Domaine is these bikes will take larger tires, fenders if need be, etc. They are arguably too light for extended touring with full bags, but they are ideal for pulling trailers, or for lighter touring. And without gear, they are plenty light/fast enough to suffice for group rides, centuries, or general fitness riding. Whereas a true touring bike like a Surly LHT feels pretty clunky for those purposes.
In this category, I personally prefer designs which take long reach sidepull brakes rather than cantilevers. Hence my comment about an older Casseroll rather than the newer ones.
If you really are desperate for carbon, at least consider some of the more practical versions. Older Giant OCR Composite frames were very light, but could take 700x28 tires and long reach sidepulls, for example.