It's bad cause you spread the gold and exp from top on two people while leaving all the gold and xp in tha jungle unused, and you also loose out objective and buff controll + the constant threat of ganks on enemy laners.
If one of your other lanes does bad, you need to comit much more to prevent the enemy from snowballing, as roaming from top to mid or even bot takes far more time than ganking from thr jungle.
So unless your other lanes play save enough to not get snowballed upon by jungleganks while your duotop denies the enemy single top havily, it will fail.
Your enemys will just outfarm and out level you over time, and if they use the jungleganks smart they will also be able to take objektives more easily.
There is a core that needs to be fullfilt for your team:(1-1-1-1+1)
atleast one member on every lane(jungle counts as lane for this)
if there are two members in one lane, one needs to be a dedicated support, which means no cs, while the other is the 'carry' that takes the farm. The reason for that is that focused gold is stronger than spreatout gold.
your team needs vision no matter where it comes from, but since a support usually gains the least gold and thus won't become much of a threat stats wise, it doesn't make that much of a difference if he picks up a sightstone instead of a item that gives more combat stats. therefor u usually pick a champion that has high impact even without much stats from items. This also counts somewhat for certain junglers.
on a sidenote, it is usually wise to have the duolane not top, since dragon is the biggest early objektive, and it's on the bottomside, so you want more people cose to it.
Sometimes you see a duolane top, but usually that lane takes the tower fast or freezes the lane while zoneing out the enemy till ~lvl 6 and than swaps with bot, cause by that time Dragon starts to become relevant so you want the two guys bot.