Neither of those guns are actually twin-linked versions of the other.
The range is different on the Bolter and Combi-Bolter.
The rate of fire is different on the M34 Autocannon and the Reaper Autocannon.
But if you consider those actual upgrades, then that means rules wise, Twin-Linking only ups the Rarity one level.
Twin-Linking a weapon does not alter either of those weapon stats.
Those are both two examples of unique weapons that have the Twin-Link quality.
"Twin- linked weapons are built this way in order to increase the chances of scoring a hit through the crude expedience of blasting more shots at the target."
Fluff wise, as written, it should get a Bonus to hit.
And also as written it has been this way, rules wise, since Dark Heresy 1.0. They have only changed it once and this was in Black Crusade, and then promptly changed it right back in Only War and kept it in DH 2.0.
Further consider the downsides.
Yes it gives a plus 20 to hit, but if I have an Accurate, Custom Grip, Modified Stock, Sniper Rifle with a Red dot, and I aim in short range, I have a +57 to hit(Single +10, Short+10, Accurate+10, Aim+10, Red Dot+10, Grip +5, Stock +2). That means, the bonus it gives, is only +3 in most common situations, where minuses are not present since there is a max +60 bonus to any roll.
Now consider you use double the ammo. Not a big deal on a single fire weapon. But on any weapon that shoots multiple rounds, you are expending twice the ammo, for one more hit(if you didn't have that +20 bonus, with that bonus, it also allows for 2 more hits in full auto and 1 more in semi auto, keeping it more balanced for the ammo used compared to a single shot.) Autogun shoots full auto of 10. If you twin-link and shoot full auto, you shoot 20 rounds for 1 more hit(If you get 2 degrees. If you don't you have the bonus +20, who would ever use it.
Increased weight is also another downside, probably for balancing reasons. It makes sense as written. It's literally just two guns put together.
"A Twin-linked weapon represents two identical weapons connected together and linked to fire at the same time, often through one pull of the trigger or push of the button."
And not that it's the biggest deal in the world, but on the note of Clip Weight. Clip weight is definitely derived afterwards at 10% of the weapons weight, However, I believe that is so you can know the weight of Additional Clips. I would argue that the weight of a full clip is already included in the weapons listed weight, being 10%. When you purchase a weapon, it comes with a full clip.
"Should it be important to know how much ammunition weighs, consider a weapon's full clip to weigh 10% of the weight of the weapon itself."