Definitely worth a try before leaving pulling one cable from the roof into the living room ...

Considering the well-known and of other reviewers already discussed "the same transponder" - or "same polarization" problem with simultaneous use of "both ends", so you can have fun with this satellite distribution. In plain English means the "problem" by the way the following: The stations must basically come "from the same satellite." So you can look for example RTL, SAT1, Pro7 & Co at the same time. RTL and MTV is not the same after the current state of affairs. I know that I am simplifying here strong, but for the average Joe-customer goes basically the statement: The standard transmitter can be simultaneously watched.
If the signal somehow (literally) remains "on track", then you should - as elsewhere described - working with signal amplifiers - and after the switch. You need then indeed for each outgoing unit a signal amplifier (two instead of one), but they are but "on the safe side" in terms of signal amplification for both devices.
Conclusion: Very good buy, saves a lot of cables, saves a lot of nerve and provides, due to the low price all as "to be tested option" to which you perhaps should try before an electrician is given the task, one cable from the roof into the to draw the living room. Knappe 10 Euro are definitely not created as wrong as the test investment.