Gosh, I don't know what parts would be interchangeable, but I'd suspect most of them are the same between the two clusters. Probably different EPROM's for miles verses kilometers. I was cruiseing on ebay in the Mercedes area and noticed a company that is selling rebuilt clusters for 129's. They have an e-mail address, so why not contact them and see if they will either repair, exchange, or use two to make one working unit. There are also other references in this forum to companies in Florida and New Jersey that repair for a flat rate, (which I think is a bit steep). But if they have investments in special instruments or jigs, I suppose they need to recoup a bit.
I really haven't a clue as to the circuitry in the cluster. But if it's a good design, it should not fail when jump starting the car. Geez, we'd be replacing the darn things all the time. I suspect the engineers may have used some protective circuitry as simple as what are called IC fuses. It's a very fast fuse that protects the IC's in the circuit. If it's just a fuse, than paying $275 for a repair ( replace the 75 cent fuse ) seems a bit much.
Resistance is Futile.