I have a level 20 orc right now. Originally, I wanted him to be a heavy armor using two-handed sword wielder, so I decided to level up my blacksmithing as much as possible, neglecting other stats. I've decided that I may want to start using light armor and take up dual wielding, despite my large advantages in two-handed weapons and heavy armor. Would it be too late to start leveling up these stats? Also, is it sensible to even use a light armor and dual wield combination? Sorry if the questions seem stupid, I'm not too familiar with TES.

I've been playing a stealth character with the build that you're considering, and it feels like a glass cannon - its very squishy. I recommend investing a lot into health, maybe even more than you currently are. You'll probably need to roll a new character, as it's been very difficult for me even on Adept setting and specializing in it from the start. Dual wielding itself is cool and effective, but only if you use the power attack, which eats heavy stamina and locks you in place for a good 3-4 swings. Also, you probably will want to carry a bow to soften things up in some encounters. The build is viable though, to get right down to it.

After playing a stealth assassin and a heavy armor sword + shield build, I'm now playing a light armor archer/dual wielder. It's a lot of fun because I'm so squishy, it forces me to use more tactics. I've given Lydia a full suit of dwenmer armor and try get her to tank while I shoot a few arrows and then run in to melee.

I'm playing on master and it's a bit challenging, I've had to reload a number of times because as soon as a two-hander swings at me, I'm dead.
Make sure you stock up on potions, and use your companion. If you try tanking in light armor + DW you will die... at least on master.

p.s I only use sneak as an opening attack, I don't stealth around dungeons like I would with an assassin.

if you use sneak/backstabb - this will get you along way, thats what i do and only engage in direct combat if i have to - but you ARE squishy so my recommendation would be to choose either an orc (berserker can compensate and fits the playstyle) or breton (additional defense - mages get NASTY cause you cant really run away from em) and also invest into alteration (additional armor and magic defense)

im playing an orc assassin - what is a big problem are boss/dragon encounter where i can not sneak up on them, cause i not really use a bow and a dragon kills me insainly fast...but its a lot of fun - and you are pretty fast! invest in the light armor perk which gets you faster and also the dualwield perks that get you faster - you can with some practice dodge the most melee blows - wait till the enemy makes a power attack, dodge and start a counter power attack your way - one problem here i always have with dualwield - you have a very short range and are locked in place for some time when powerattacking, the problem is that you often might not hit with the powerattack when the enemie moves...

but for such a build your most concern should be - facing several enemies at once (especially a combo of melee and ranged) or boss monsters (can be partially offset by investing in conjuration oder teaming up with a companion)

My character is pure combat dual wielding/light armor. I can kill dragons in 5-6 seconds, and everything else dies just about as fast. The only difficulty comes with enemies that do absurd one hit damage, such as giants, draugr deathlords and dragon priests. However, if you hit those with a fully charged Fus Ro Dah, they go down for several seconds; well enough time to do absurd amounts of damage. I can kill a giant without them getting up even once, deathlords are a bit harder, and priests harder still, but they are the ONLY challenging fights i've had so far in the game.

The key is focusing on doing as much damage as you can, as fast as you can. The better you do at that, the better you WILL do at that, because all your XP will go to single-handed, letting you level up your single-handed just that much faster.