well they won't fit side for side, so a portion of your rad will not be covered by any fans. Why not just use the proper fans for the proper rad?
Assuming it is a "cone" style adapter, then 10mm will be hanging over the rad as well, which may not work for most cases that have a 360mm rad spacing, but then even so, you will have 260mm of horizontal coverage on a 360mm surface. 1/3 of your rad is not being cooled. I do not see it being effective in any way and probably worse for cooling.