No easy way to do this in ST Routines because there is no way to read the value of the kitchen dimmer and apply it to the sink dimmer (no variables). There are some ways to synchronize behaviors using either association groups or the Smartlighting app, but that might not be what you are really wanting.
You could probably achieve what you want if you install the Aplicaciones Virtuales Mc driver Applications: Number Fields and Basic Calculations which allows for storing values (and doing simple calculations on them) and using the Rules API. After installing this driver, you would create a rule that writes the dimmer value to a field when the kitchen light is turned on. Then when the sink is turned on, you could set the sink dimmer to the value you wrote into the field. I’m personally not well versed in using the Rules API, but someone like @TapioX might be able to give you an example.
Alternatively, you could pick some pre-defined dimmer values and set the kitchen and the sink dimmers to come on to those values based on time of day or some other criteria using Routines.