A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Togashi Kazuki Open: Target another of your Personalities. Set his Force to Kazuki’s Force or his Chi to Kazuki’s Chi.
Kazuki currently has 4F and Togashi Bob has 1F + 1F from a weapon. I use Kazuki's open action to set Togashi Bob's force to 4. Does Togashi Bob then get an additional +1 from the attached weapon?
Togashi Kazuki Open: Target another of your Personalities. Set his Force to Kazuki’s Force or his Chi to Kazuki’s Chi.
Kazuki currently has 4F and Togashi Bob has 1F + 1F from a weapon. I use Kazuki's open action to set Togashi Bob's force to 4. Does Togashi Bob then get an additional +1 from the attached weapon?
No. His current force before Kasuki's bonus is 2, so he gets a +2F bonus to match Kazuki's 4 Force.
I'd recommend you transfer the +1F weapon to Kazuki first. Then you'll wind up with two 5F Personalities instead of two 4F Personalities.