Be aware that UAV writes from the Pixel Shader will disable any 'Early' (pre-pixel shader) depth testing unless you annotate your shaders with [earlydepthstencil].
Depending on what your "special conditions" are and how far in advance you know whether you will/won't want to write the second target really determines what the best approach is. If you know before you issue the draw call whether you're going to want to write to the second target or not then it's simply a matter of compiling two versions of the shader; one with one RTV write and one with two (or a single shader with both and then unbind the second RTV, but this isn't ideal).
If you need to make the determination whether to write to the second target or not only on a pixel-by-pixel basis then alpha blending really is the best approach. What is the 'struggle' here? Setting up a blend state seems less difficult than adding UAV writes to your pixel shader.