Ohms are a measure of resistance.
Amplifiers are rated based on the maximum amount of power that they can deliver into a resistive load on a continuous basis without clipping the signal. Clipping occurs when the amp cannot produce enough voltage to fully reproduce the signal.
Speakers are rated by their impedance, which is a form of resistance which varies by frequency. While many manufacturers do not adhere to the IEC standard, it states that a speaker's actual impedance should not drop below 80% of its rated impedance. So, for example, there should be no point on an 8 ohm speaker's impedance curve at which it drops below 6.4 ohms.
The amplifier must deliver (or try to deliver) the current demanded by the load (ie the speaker). The lower the speaker's impedance, the greater the amount of current it draws. Read up on Ohm's Law.
Theoretically, if resistance is halved, current is doubled. In the real world, most amps cannot produce enough voltage to double their delivery of current. To do so, the amp must have a fully regulated voltage supply (power supply), and enough output devices (output transistors) and heat sinking to withstand the accompanying heat that is produced. The heat is as a result of inefficiency in the conversion of input power to output power. The power supply, output devices and heat sinking are the most expensive elements of an amplifier, so to keep them affordable, compromises are made.
This is reflected in your amplifier's power rating. Ideally, if it produces 65 watts into 8 ohms it would produce 130 watts (voltage x current) into 4 ohms.
So, again speaking about an "ideal" amplifier, the power going into two pairs of identical speakers is the same as is going into one pair. The load presented by a pair of 8 ohm speakers wired to the amplifier in parallel = 4 ohms. So, if the ideal amplifier is putting out 2 amperes of current into a single 8 ohm speaker, it would put out 4 amperes into the 4 ohm load presented by the two paralleled speakers,, with each getting half of it. Voltage would be unchanged (theoretically), assuming same setting on the volume control, so each of the 2 paralleled 8 ohm speakers would get the same amount of power that a single 8 ohm speaker would get. BTW, the two speakers would produce about 3dB more "loudness" than the single speaker.
The Advents, if rated via IEC standards, would be 6 ohm speakers. A pair of 6 ohm speakers = 3 ohm load. Most consumer amps, including yours, are at best rated for a 4 ohm load. The harder your drive them (the louder), the more stress you are putting on the amp. Your Sansui will probably acquit itself quite decently against this challenge, but just keep in mind that you are asking it to perform at/slightly beyond its limits.
