I am not sure that I have understood you correctly. If yes, I can propose a workaround. The idea is to add a third list Range[0, 10]. For the first two you specify Joint->True, for the third Joint->False. Further, you specify the legend for the first two, and Legend->None for the third. Try this:
colors = ColorData[97, "ColorList"]; fontsize = 20; leg = LineLegend[{colors[[5]], {Black, Dotted}}, {Style["simulation", FontFamily -> Times, fontsize, SingleLetterItalics -> False], Style["experiment", FontFamily -> Times, fontsize, SingleLetterItalics -> False], None}, Spacings -> {0.4, 0.08}, LegendLayout -> {"Column", 1}, LegendMarkerSize -> {22, 10}, LegendFunction -> (Framed[#, FrameStyle -> Directive[Black, Thickness[0.5]], RoundingRadius -> 5, Background -> White, FrameMargins -> -1] &)]; plotbend = ListLinePlot[{Range[0, 10], Range[0, 10], Range[0, 10]}, PlotStyle -> {colors[[5]], colors[[5]], {Dotted, Black}}, Frame -> True, AspectRatio -> 1.1, Joined -> {True, True, False}, PlotMarkers -> None, BaseStyle -> {FontFamily -> Times, FontSize -> fontsize}, FrameStyle -> Black, FrameLabel -> {Style[ "Normalized curvature, \!\(\*FractionBox[\(\[Kappa]\), \ SubscriptBox[\(\[Kappa]\), \(0\)]]\)", FontFamily -> Times, FontSize -> fontsize], Style["Normalized bending moment, \!\(\*FractionBox[\(M\), \ SubscriptBox[\(M\), \(0\)]]\)", FontFamily -> Times, FontSize -> fontsize]}, ImageSize -> 360, PlotLegends -> Placed[leg, {0.73, 0.14}]] returning the following:
Hope it helps.