Inkscape; how do I add end markers for two lines that are combined?

I've been creating a diagram on folic acid metabolic pathways in mammals and I have two lines I've grouped together, that I'd like to give end markers to but for some reason only one of these two lines has an end marker. I've tried adding a start marker (as I was wondering if Inkscape was counting the other line as the start of the overall path) and a centre marker (out of desperation), neither worked.