Marco, You are indeed doubling the load to 4 ohms, nominally. Most [tube] amps [if that's what you are running] are not well-equipped to drive such loads, 4- ohm taps notwithstanding. Even many ss amps don't like low impedance loads. Of course, the proof is in the pudding, or listening, in this case.
Niles probably runs an interstage xfrmr for such purposes. Not all xfrmrs are bad, but the Niles' first priority may not be sound quality. Better to procure an amp for the second pair, although gain-matching issues come into play unless amps and spkrs are respectively identical. The Hafler "quad" ckt may be an option. Oops, guess not if one pair is downstairs. I just imagined the horror of 50 foot cables, too long for any sane app. Sorry if I raised more ?S than answers, but that may go to the root of your problem. Best is two spkrs, one amp, imo. Perhaps a condom next time?
Niles probably runs an interstage xfrmr for such purposes. Not all xfrmrs are bad, but the Niles' first priority may not be sound quality. Better to procure an amp for the second pair, although gain-matching issues come into play unless amps and spkrs are respectively identical. The Hafler "quad" ckt may be an option. Oops, guess not if one pair is downstairs. I just imagined the horror of 50 foot cables, too long for any sane app. Sorry if I raised more ?S than answers, but that may go to the root of your problem. Best is two spkrs, one amp, imo. Perhaps a condom next time?