The MLC truly is the maximum carry on, you can put a lot of stuff in there (as much as your shoulders can bear). I have replaced the shoulder strap with one of these which are the same as the much recommended Tom Bihn straps.

I was a little bit surprised by how big the MLC actually was upon receiving it , you are totally right, a carry on and an MLC is totally pushing it to the limit. It may or may not work depending on the gate agents, and LH tends to be very strict even on international.

The reason I intended to do this in the first place is because LH has their staff walk the boarding area harassing passengers and actually weighing carry on items on international flights, their limit is a ridiculous 8KG (17LB) and some wheeled carry ons are up to half that weight. Of course nobody checks their bags but just shuffles stuff around. In any other airline you could take a ship anchor inside and nobody would care.

On a week trip with no access to laundry in cold weather and not wanting to check bags to make things faster, you see how the MLC makes sense.

I might just end up taking the MLC alone or a wheeled carry on and a small attachable personal item to shift weight around/bring back something. Any recommendations?

The Eagle Creek you mention is 13 x 20 x 8 in (41 liters), and the MLC is 20.5" x 13.5" x 7" (45 Liters), not much of a difference.