[MapProxy] Idea: Combining Caches

[Marc Harter]

It seems to me that you can only request one layer for a WMS service at
a time but say I have a cached aerial photography layer and a cached
vector layer, could there be a way to request multiple layers and have
them fused into one image dynamically instead of having to build three
caches, (one vector, one photo, one both)... could build two.

e.g. ?SERVICE=WMS&LAYERS=cache1,cache2&...