Just bear in mind that connecting the 2 consumer units together is a fairly simple task in its self, and if you get 2 wires in the right place, or even almost the right place, everything will work.
What you won't know until it happens, is how the installation will behave in the event of a fault. If the cable linking the 2 consumer units is not correctly protected from overload and fault current, then there is a risk of fire. If it's not suitably mechanically protected and in the wrong place, there is a risk of someone getting a Shock when they drill through it etc.... And if the supply isn't adequate it will keep tripping out/going off. There are also issues with the type of earthing and how and bonding conductors for the upstairs dweling are connected.
And whilst you may be aware of these downfalls, it probably won't be your property forever and you won't be the only user.
Getting that part right requires proper design based on the individual circumstances of the job. And people could spend a lot of their time working some of it out via the internet, but are often reluctant because that is how they make a living.
There are also a number of tests which should be carried out once the cable is installed, to ensure it's safe prior to energising, and even time served electricians may pick up on mistakes during this testing process.
These are some more of the reasons why you have been advised to seek a professional for this task.