You should only need one antenna, you can split the antenna signal. (there will be some signal loss.) You can buy antenna amplifier but I find they just amplify the noise as well.
You can split the decoder output and connect to two tv's.
I'd first put in the antenna and connect to your current decoder.
If all is great (20 channels) then split the signal to the other tv.( split in the attic.)
Buy another decoder box. Enjoy