Here are two possibilities:
1) Do you really need two separate server blocks? The simplest solution might be to set an entity attribute based on what server it was processed by (75% chance Server Attribute is A, 25% chance it is B). Then you can simply use a Set Attribute block and a single Server block (note service time can depend on the attribute so you can set different times based on the server). No need for any routing blocks at all in this case.
2) If you really need two separate blocks, you can use the routing blocks you've set up. In order to only allow one entity in either server you can put an Enabled Gate before the Output Switch. For each server output the number of entities in block (#n), add those two signals together (Add block), then use a Compare To Zero to see when that signal equals (==) 0. Convert that signal to double using a Data Type Conversion block, then send that signal in the enable port on the gate block. So basically you will open the gate only when there are no entities in either server block.
