My two tandeming anecdotes:
1. When I worked in New Zealand for a while, a friend of mine had a home-made tandem cobbled together from two bikes. We used to use it as a pub bike which had the advantages that it was unlikely to be stolen; and that the most sober one could go on front. I remember easily hitting 30mph on the flat ground of Christchurch.
2. My girlfriend at the time and I once took a tandem to Arran and we left the car behind on the mainland. Unfortunately my dad had fitted the wrong size of inner tube to the front and the tyre blew out on the descent into Lochranza. It took all of my skill and about 500 metres to bring the bike to a halt and not crash, as I couldn't use the front brake much, the rear was fairly useless and the bike was wobbling about all over the place. Luckily the GF was fairly oblivious to the peril.
We walked down the hill into Lochranza and found accommodation for the night. The next day, the bike hire place in Lochranza refused to sell us an inner tube so we had to wait hours for the post bus to bring one from the main bike shop. Once the bike was finally back on the road, we decided to abandon the trip as it was pouring with incessant rain, and pedalled back over the hill to Brodick.
Unfortunately GF decided she was not able to stand up while stoking and also was not able to take control on the front. This meant we did not have the horses (nor the gearing) to get up the hill and ended up walking it in the pouring rain. Needless to say she was not a happy camper. It did not end in 'divorce' but it was close...
So, the advantage of two solo bikes is that if one breaks down, there is still another to go and get help (unless you crash into each other).