The signal isn't consumed, as such. It doesn't matter whether a TV is on or off or not even connected to an output at all, a simple splitter divides the signal between the outputs regardless of what's connected or not.
A basic passive (non-amplified / non-powered) 2-way splitter divides the signal in two so there's half on each output, and there's a small additional loss from having the signal pass through the splitter circuit itself. A typical 2-way splitter then will reduce the signal level on each output by roughly 3.5dB, and that equates to approx 45% of the original on each output leg.
Since your roof aerial has two cables coming from it, and (we hope) it has been split properly rather than bodged, then the same reduction of signal level applies to it too. IOW, you're starting off with a bit of a handicap because you're getting less than half the signal that the aerial can provide. You should bear this in mind if you come across issues of intermittent signal loss or pixilation.
Your next question is probably something like "What about if I join the two cables?". IMO that's a bodge and it runs the risk of creating odd issues such as constructive and destructive interference because the signal path lengths are probably not the same length. I'd leave it as it is, but probably fit a 75 Ohm terminator plug on the end of the unused leg just as an insurance policy to help prevent noise. It certainly won't hurt, and at under £3 it won't break the bank either.
Thinking about your aerial, if it has been up for any length of time then there's a good chance that it's no longer the best choice for optimum reception from your local transmitter. This is happening a lot over the UK. All the Freeview retunes are about moving the transmission frequencies to the lower half of the old channel band so that the upper part can be sold off for mobile phone use.
We used to have a frequency range from around 470Mhz to 850MHz - or RF channel 21-68 if you prefer. Now it's 21-60, but reasonably soon it will be ch21- 48. Most wideband high-gain aerials sold for analogue and digital had their best performance between channel 50 and 68; that was where they were high-gain. Out of that range - in the lower frequencies - those aerials' ability to drag in signal falls off quite rapidly. They still work, but nothing like as efficiently, and that means less signal and more noise. Noise is the enemy of any amplified aerial distribution system. Combine this with the halving of the signal level because of the two cables and you can start to see where you might get a few glitches where the reception gets weak.
Powered (amplified) splitters give us the ability to have more outputs than a simple passive splitter might support given the strength and quality of the aerial signal. They come with a couple of catches though: First , the amplification doesn't fix a weak signal from an aerial. If the signal going in to the amp is bad, then all the amp will do is make a small bad signal in to a larger bad signal. It won't turn a bad signal in to a good signal. No domestic aerial amp can pull that trick off.
Second, any kind of powered amplification adds noise to the aerial system. Noise is the enemy of signal quality, and signal quality is by far the most important facet of any aerial system.
So, what to do?
Well, my gut feeling is that your aerial signal won't be strong enough to stand up to passive splitting, so you'll need an amplified splitter. The amount of amplification doesn't need to be huge though. All you really want to do is compensate for the signal level loss in the cables going down to each room. Good cable (Webro WF100 and similar, not stuff just sold as 'RG6' or 'low loss coax') reduces signal strength by 1.5dB per 10 mtrs. A gain figure of 5-6dB is plenty for most applications. Some amps such as the Wolsey 334014 (look on CPC) have adjustable gain. In this case it's 0-10dB. That's handy if you get the aerial sorted at a later date and find you need less power from the amp.
My personal preference is an amp that uses the screw-on satellite-type connectors. They hold better and tend not to work loose over time. They're also easier to fit than TV coax plugs.
Where you're buying an amp that has either an FM/DAB input or a Full Level output then make sure to also order some 75 Ohm terminator plugs. They'll reduce noise for the unused radio aerial input, and they'll improve the signal level from the ordinary outputs in the case of terminating the Full Output.
An amp shouldn't cost more than £40 typically. Run this and then if you find you're having problems with certain channels it's time to get an aerial guy out to replace the roof aerial with something better suited to the local transmitter and where signals are heading.
