So basically, I want to make a desk that is 8 feet long of MDF with a veneer top and hardwood trimming, I haven't worked extensively with MDF before and I have read a lot complaints about sagging over time.
My current design is 24" x 96" MDF that is two 1/2" thick slabs of MDF glued together with two 8 foot steel .75" square 16 gauge rods running up and down the length.
Will this sag length or width wise?
An 8 foot long 3/4" steel rod will sag, rather dramatically, all by itself.
The same amount of steel in a different shape (tubing or angle, typically at this scale) would be much stiffer. As vaguely described, (I can't tell if you are putting the steel between the layers, beside them, or under them from your description) your desk will still sag.
A far stiffer basic slab structure is to build a "torsion box" with thin skins and lightweight spacers between the skins. It also happens to end up a lot lighter than a solid hunk of MDF.
I suggest you stop by the steel store and give that bar a sit. In other words, if it doesn't support your weight fairly effectively at the center of an 8 foot span it's not suitable for a desk of similar length. You'll find that not only is 3/4" square rod heavy, it's quite flexible. With steel, as with any material, shape matters. You need height, not weight.
Take a look at the many desk discussions on this site. The nearly all have this same flaw in their designs. You either need intermediate support from a wall, or substantial longitudinal beam structures. Consider a bed frame as a reasonable guide. Most have either 2" steel angle or ~6" wooden side rails, if not legs in the center.
It will be very heavy and the strength comes from a supporting structure, not from the MDF. I would make the frame out of timber and then line it with MDF if required. And you always have to worry about the MDF coming into contact with water and turning into breakfast cereal.
This is from my experience in building couches for ourselves when we purchased our first house. We could not afford to buy carpet or furniture. So I built it as above. It was extremely heavy. And it lasted for over a decade until we had a house fire when it turned to mush from the water used to put it out.
