Double deck trains - again ...

[HSTEd]

In these cramped double deckers, where is all the luggage going to go, bearing in mind that virtually everyone on these services travels with at least some item of luggage, and many people carrying plenty?

A TGV-M will not be significantly more cramped than a Class 373.

Would the TGV-M be able to operate in pairs through the tunnel or would it need to be a single 400m train? If the latter, how would that work with the power cars?

HS1 and LGV NOrd are pretty low performance high speed railways by modern standards, they only require 300km/h operation.
A pair of TGV-M power cars, especially if the adjacent trailer bogies were powered as on the Class 373, would probably be able to move a 400m TGV-M trailer formation at the accelerations and speeds required. A double deck TGV trailer isn't much heavier than a single deck one.

But I think the TGV-M proposal for Eurostar is for 200m sets, although that is obviously not guaranteed as the order has not actually been placed yet.