The Extended Monotropic Programming problem deals with a more general extension to all n variables at a time than the two variables at a time extension you are interested in, except that the literature I have seen deals only with the constraint set $C$ being a closed linear subspace, not a general convex set.

Specifically: 

$\min_{x \in \text{closed linear subsapce}}  \Sigma_{i=1}^nf_i(x_i)$ with respect to $x$, where $x = (x_1,...,x_n)$, $x_i \in \mathbb{R}^{i}$

See ["Extended Monotropic Programming and Duality", Dimitri P. Bertsekas, 
Journal of Optimization Theory and Applications volume 139, pages209–225(2008)][1]

2010 postprint available at https://www.mit.edu/~dimitrib/Extended_Mono.pdf

Additional reference:

[On a zero duality gap result in extended monotropic programming,Radu Ioan Bot,  Ern ̈o Robert Csetnek"][2]

Older published version: https://link.springer.com/article/10.1007/s10957-010-9733-y



  [1]: https://link.springer.com/article/10.1007/s10957-008-9393-3
  [2]: https://arxiv.org/pdf/1002.3272.pdf