You most certainly would degrade power by adding a splitter in series. Unterminated ports would also be points of noise ingress.
I take it you want to move your TV box around to different TVs - something like that.
Presumably you've looked at
http://mediacables.co.uk/shop/index....ain_page=index and reached the conclusion that you need to put a splitter in series.
If your input downstream power is reasonably high (like not below 0 dBmv) then a series splitter with unused points terminated should work OK. You've then got the issue of unterminated coax leads to consider! So you'll need male and femal terminators.
Hope thathelps.