first, black layer mask on dark one, white brush the sky(over the horizontal line), lumenzia selection on the ground, black brush the horizontal line. if there is white halo, exchange white and black brush several times, or add a brightness adjustment layer between the bright (foreground) and the dark (background) layer, lower the brightness, or (add a new layer, change the mode to darken, use the stamp tool, pick up the nearest sky color, brush on the halo edges. )
or , you add black layer mask on dark one, white brush on sky (not over the horizontal line), use quick selection tool to select the sky area, white brush on the horizontal line, if there is halo, exchange between black and white to erase halo.
remember: white reveal and black conceal. also you can memorize as white has influence.