Complex sentence: Marstrand and Hansen submitted their decoration proposals in 1861, having decided that Marstrand would paint the paintings while Hansen would paint the framings, and in the same year the pair travelled to France to study oil painting on plaster.
Split into simpler sentences: Marstrand and Hansen submitted their decoration proposals in 1861, having decided that Marstrand would paint the paintings while Hansen would paint the framings. That year the pair travelled to France to study oil painting on plaster.