Hi. We've done over 3000 miles on a set of Gatorskin Hardshell 700x28c running at 100 psi on our summer tandem and on faster rides in winter. The winter tandem (used for more rufty tufty rides) has done over 5000 miles on Marathon Plus 26x1.5 running at 60 front 70 rear. Both sets have performed faultlessly. I hope this helps.

What size wheels are you using? You have named the two current favourite tyres among tandem types, I have used both manufacturers tyres in different circumstances, and I think after some years of reflection I know why I prefer gatorskins in 700c. On 26 wheels there is one "small" (about 32 to 35mm dimensions) size gatorskin available, I only use them on road, ridden at very high pressure on tandems and has great cornering grip - but is nothing like as comfy as the 50mm tyres available in 26 which I prefer as they are also viable on trails and tracks.

I used marathons then super m for about 7 or 8 years in 700c and 26, but slowly got fed up with the hard ride and poor grip on bends. I used XCR for a while but had 3 of 4 tyres with the rim wire breaking - found two stored one like it after one tyre demounting and tube exploding on a ride. Worrying, but Schwalbe UK sent free replacement tyres in replacement.
In 700c marathons on tandems and solo use were really draggy and harsh compared to the conti tyres. I didn't notice the same problem in 26" size but only have limited expoerience of the super marathon in 26 as for various reasons I sold on my bikes fitted with them. I now use only 700c gatorskins on my solos, they are great.

I think it is the size of the air pocket on the plus that is the problem. Schwalbe have very hard sidewalls, conti make theirs very flexible, even the hardshell is much more supple than a schwalbe tyre. When the marathon plus came along I tried 700x28, but with 5mm foam reducing the inner air pocket volume of the casing by 20%ish I had to fit 23/25mm innertubes - the air pocket inside was so much smaller than a normal 28mm tyre that tubes were a pig to fit without pinching in 28mm size. Not surprisingly the super marathon tyre with a hard sidewall and much smaller air volume rode much worse than the conti with a traditional design. I live in a city centre so the start and end of every ride has 4 miles plus of urban glass to contend with but the puncture rate for both tyres was so occasional as to be irrelevent as a deciding factor. I have shredded two gatorskin sidewalls in 10+ years and they seem much more prone to impact damage.

The last 700c non hardshell conti I rode on a tandem disintegrated, the plies de-laminated in several spots inside the tyre and it bulged so much it would no longer rotate when inflated - on a ride at the TC rally in the Czech Republic. In fairness it was a well worn rear tandem tyre but I was shocked at the state it got in to. I fitted hardshells after that which were fine but have since sold on the tandem they were on before many miles.

I am using Dureme Tandem tyres (now unavailable) on my 26 wheel tandem, run at their recommended 70psi max pressure. These have double thickness sidewalls but no internal foam/gel strip so the tube has a huge volume of air and the ride does not seem harsh compared to other 2" 26 tyres. No punctures yet but not many miles either.

In summary my opinion is Schwalbe in 26 and gatorskin for 700c.

I hope this epic in navel gazing tedium helps.