Now they practically mean the same–both (i) a day, and (ii) the sun. But if we look back far into their history, as found in ancient writings, we see the convergence of the meanings took place relatively recently.
The Old Meiteilol (OM) কোয়লৌ was the compound of কোয় (circular, round) and লৌ(বা) (to regard as, to take as, to assume). More than it seems to us now living among tall buildings, the unlimited sky (because of the limited vision of us humans) looked to the ancient Meiteis like a dome, arching over them like a cage. Thus, they took/regarded/assumed (লৌবা) sky as domed, circular (কোয়বা), that is, কোয়না লৌবা, in Meiteilol. The sky was bright and that was the day. The sky or the day was bright because of the sun. It was the sun that made the day/sky visible; that is, visible as domed. The domed look of the day or the sky was basically ascribed to (regarded as of) the sun. Moreover, the sun was a disc–circular. That made the connection easier. That is how কোয়লৌ came to mean both the day and the sun.
Humans love it short and simple, and our tongue, despite its boneless flexibility, wants to spend as little energy as possible. This leads to dropping of sounds, shortening words, and assimilation and sound change looking for comfort (a form of economy) in saying words. Thus, in course of time, the OM কোয়লৌ /koìləù/ dropped the vowel /ì/ giving কোলৌ /kòləù/, and following the Bangali influence beginning in the first quarter of the 18th century, the alveolar lateral approximant /l/ changed to the alveolar trill /r/ giving কোরৌ /kòrəù/.
A younger word, নুমীৎ /numìt/ has a different story, and it is rather more physical and philosophical than linguistic. নূ /nù/ (it was not নু /nu/) was the common gender for human in OM. Add the masculine and feminine gender suffixes (-পা and -পী respectively) to this, and you get নূপা (man) and নূপী (woman), respectively. মীৎ, as it is today, was the eye. নুমীৎ thus meant the human eye. However, there is still a twist. The ancient Meiteis believed that the eyes could see because they were নূ (humans), and humans had some inner power that could make the eye see. The even subtler philosophical or scientific twist is this–the ancient Meiteis believed that humans had that inner power making the eye able to see because the sun shone up there in the sky. The light of the sun makes humans capable of seeing. Thus in the final analysis, it was the sun that was the eye that had the power of seeing and the power of the daylight (day + light). It follows that it is the eye (the sun) with which we see.
This is how the word নুমীৎ came to mean both the sun and the day.
As we see what we have is নুমীৎ /numìt/, not নূমীৎ /nùmìt/. It is because a sound change from /nù/ to /nu/ occurred in course of time. We are not sure when it happened.
An interesting addition:
Meiteilol has been down the history for more than two thousand years, I guess. Time adds layer to everything–to the earth, to trees, and also to languages. Thus a language often has old and new words for the same concept, and most languages use either of these words according to the contextual demands. Though the context matter still matters in Manipuri, we have a slightly different and interesting case here: we often use both the old and the new terms (usually it is the old one that leads the compound) together. কোরৌ and নুমীৎ combine to form the compound কোরৌ নুমীৎ, to mean both the sun and the day. The context specifies what it exactly refers to in a specific use. Similarly, we have লাইজা ঈশিং, লেম্লৈ ঙা, নোংদোন অতিয়গা, among several others.