So, why do Kagyu followers visualize the late instead of the current holders of the Karmapa office? Does this have to do with the status of the late living Buddha as opposed to the living living Buddha?
Maybe because there's yet any conclusive resolution to the Karmapa controversy? But anyway, just to clarify, you probably meant living Tulkus, not living Buddhas, for there can't be multiple Buddhas existing in the same world system. After Gotama Buddha's passing, and after a long period, the next Buddha will be Metteyya, who's currently carrying out His bodhisattvic duties up there in the Tusita heaven. All others will be just teachers with various degrees of attainments from now until that time.
“He understands: ‘It is impossible, it cannot happen that two Accomplished Ones, Fully Enlightened Ones, could arise contemporaneously in one world-system—there is no such possibility.’1089 And he understands: ‘It is possible that one Accomplished One, a Fully Enlightened One, might arise in one world-system—there is such a possibility.’ He understands: ‘It is impossible, it cannot happen that two Wheel-turning Monarchs could arise contemporaneously in one world-system…It is possible that one Wheel-turning Monarch might arise in one worldsystem—there is such a possibility.’ ~~ MN 115 ~~