mergeFunc = x [Function] {Total[#], Union[Flatten@#2]} & @@ (x[Transpose]);

Merge[{<|"effect" -> {5, {2, 3}}|>, <|"effects" -> {4, {1, 3, 5}}|>}, mergeFunc]

Merge[{<|"effect" -> {5, {2, 3}}|>, <|"effect" -> {4, {1, 3, 5}}|>}, mergeFunc]

<|"effect" -> {5, {2, 3}}, "effects" -> {4, {1, 3, 5}}|>

<|"effect" -> {9, {1, 2, 3, 5}}|>