I might have some information soon on this. I might be using the DL AMEX 2-1 certificate for a PHL-LAS trip in April. From what I hear at least my paid ticket should be in an upgradeable fare class, so I'll see if I can use my MSU's for that ticket. I've read where in some cases Delta books the 2nd ticket in the same class, instead of some special fare class, in which case I might be able to use her MSU's to upgrade my travelling partner.
I should know within a week or so, I'm hoping the fares come down a little, but right not the DL 2-1 fare actually looks to be the cheapest.
Jeff