If you've got a
strong signal, a passive splitter can do it with no adverse effects to either TV. With the old analogue signal, passive splitting often led to snow / grain - but with digital it will either work perfectly or it won't (without break-up).
Cheap enough to try it first, with this and 3 'F' plugs:
If that doesn't work, you could try a 2-way booster - works fine for many people - but (over)boosting digital TV feeds can do more harm than good - might be worth getting one with variable gain.
If you've got access to the aerial (or at least the lower end of a long mast), the 'best' way to do it is with a powered 2-way masthead amp. (The supplied plug-in PSU puts 12v. D.C. up the downlead to power it):
2 WAY 1-20dB VARIABLE GAIN MASTHEAD AMP KIT. 2 Way Masthead Amplifier with F type connections. 13 Amp mains plug connected. Output Level (dBuV): 2 x 85. Max Current (mA): 200. Outputs: 1. Connectors: F.
www.ebay.co.uk
Hopefully the first, cheap way will be all that you need.