These are 2 different sets of adjacent ruins with two separate 2.50 eu. entrance fees. You can basically see right through the bars of both these sites by just walking past them on the serpentine streets of the Plaka starting hard by Monastiraki Square. Merely turn right upon exiting the street level Monastiraki Square station. You will immediately come to what's left of the Library of the Roman Emperor Hadrian - the same emperor who built the Arch and a lot more besides. Hadrian was a Phillhellene and endowed Athens with a lot of civic works including Hadrian Street street signed as Adrianou. You cant miss it if you walk the street of the Plaka!
Pictured below is part of what survives of The Library of Hadrian. Vendors often line the street because if you turn left instead of right (towards the library) you come immediately to the Monastiraki flea market. You should walk & shop this whole area!