This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 昍 is uncertain. It visually consists of two 日 (sun) components placed side by side, but its original meaning and formation are unclear.
Two suns 日 side by side suggest intense brightness, but the actual meaning is uncertain. Think of it as a double sun, perhaps emphasizing light or clarity.
For ケン, imagine a ken (a range of sight) so bright that it needs two suns: ken -> ケン, and the double sun 昍 lights up the whole view.