Should a kohen make a beracha on his talis koton on a day of birkas kohanim (or in Israel every day) if he will be making a beracha on his talis for birkas kohanim (during chazzaras hashatz)?
The question is that MB’s reason not to make a beracha on talis koton and just patur it with talis gadol (even when the talis gadol will be worn a long time later) might not apply when he knows affirmatively that his talis koton is kasher… How do we paskin l’maaseh?
If the kohen is going to make a bracha on the tallis gadol that he puts on in shul then he is like any married man that wouldn’t make the bracha on the tallis katan.
Regarding the reason that you mentioned, I don’t really understand you. The Mishna Berura (8-24) gives two reasons for the minhag not to make a bracha on the tallis koton when the person will make a bracha on a tallis gadol later. The first reason is because sometimes the tallis Gadol is put on right after the talis katan and it causes a bracha l’vatalah. The second reason is because there are times that the tallis katan is not kosher. These are some of the reasons for the minhag. (See Piskei Teshuvos sman 8 ftnt. 143 for more reasons). Therefore even if he knows that his tallis katan is kosher there are other reasons.