Topband: Adding a yagi to a 160m 1/4wl vertical

[n4uk@mindspring.com]

      Hello to all!         
          I am going to put up a KLM 40M2 2 element linear loaded 40m yagi (16 foot boom) on the top of my Rohn 25 1/4wl 160m vertical in the next couple of weeks.The tower is 115 feet high with 7 feet of mast sticking out of the top. I am going to lower the mast so only enough is sticking out of the top to support the yagi to offset some of the toploading from the yagi. I know this is going to screw things up for my 160m antenna due to the top loading of the 40m yagi and that I'll need to re-tune the whole thing.I am also aware that I'll need to choke off the 160m R.F. on the feedline of the 40m antenna and the rotor cable. Has anyone else done it backwards like I am doing? Any other caveats I need to be made aware of? What kind of effect might this have on the performance of the 160m vertical? All insight welcome.
73, Ken, N4UK