See the attached file. I changed the way the mullions are stretched. You can have multiple actions working with just a single parameter and then use a 'distance multiplier' for the different stretch actions which you can find in your properties toolpalette. The trick here is to use fractions when filling in the numbers, if you open up the block editor and select one of the strech actions and look in the properties you will find the 'distance multiplier' one of the reads 0.6667 and the other 0.3333 but when you type in those values use fractions (so 2/3 and 1/3) Autocad shows the values as decimal but internally it keeps track of them as 2/3rd's and 1/3rd.
When you need to keep something centered then using 0.5 works just fine. You just need to be carefull when selecting which objects are 'stretched' if you select the same item for multiple actions then it will have ALL the stretches applied to it.
WINDOW.dwg