Help simply the resistor usage in this voltage summer and divider circuit
simulate this circuit &ndash; Schematic created using CircuitLab

To  get 0.5V amplitude from a 5V signal  r8||r9  should be 100k/9 
I made a spreadsheet to solve that and picked values for R8 until I got close

but as you don't need both signals at the same time it can actually be simplified even further.







simulate this circuit