Adjust oven rack to middle position and heat oven to 375°.
Spread lemon curd into par baked and cooled tart crust. Set tart pan on baking sheet and bake until filling is shiny and opaque and center jiggles slightly when shaken, 8–10 minutes, rotating pan halfway through baking.
Transfer tart with baking sheet to wire rack and let cool 10 minutes.
Combine rhubarb curd and ground freeze dried berries, if using.
Lightly spoon rhubarb curd over top of lemon curd and, using an offset spatula, gently spread curd into an even layer, smoothing top. Bake tart until set and center slightly jiggles, 15–18 minutes.
Let tart cool completely, at least 1 hour, before serving.
Store tart in refrigerator for up to 3 days.