What I mean by "two tape decks" is that I have my "main" tape deck (my Tascam 34b) and a separate, 1970s Akai home stereo open reel tape deck, which I use for my tape effects. My primary tape deck is there just to record the effect created with my "auxiliary" tape deck. To create a really cool (or just weird) near-infinite tape echo, you create a feedback loop on the auxiliary tape deck (its output is fed back to its input) by way of your mixer. Depending on the effect that you want, you'll likely need to attenuate some arbitrary frequencies on the channel that you're piping the auxiliary tape deck's output to.
On
my Tascam M-30 mixer, I might do this:
Instrument on channel 1, assigned to Buss 1.
Buss 1's auxiliary out goes to auxiliary tape deck's input.
Auxiliary tape deck's output is plugged into channel 2 on the mixer, and channel 2 is assigned to Buss 1. Roll down the EQ at 12.5k, and usually somewhere around 6-8k (this is to avoid really nasty squeals), and set channel 2's fader slightly lower than channel 1's fader.
I'll sometimes add a compressor/limiter on to Buss 1 to keep the feedback from getting too out of hand in case I make a mistake in EQing the signal... to keep the meter from pegging.
Think of old
Dub tracks from King Tubby or whatever. He and other dub/reggae artists sometimes pushed the feedback pretty far, but if you adjust the levels a certain way it can add a nice echo trail to whatever it is that you're inputting into your mixer.
I don't know if I'm over-explaining this - you might've been confused by wrote because it could have sounded like I was using two separate tape decks in cooperation to create some kind of kooky tape effect.
(Added in edit) Also, I use the auxiliary tape deck to create an effect that is similar to reverb. If the instrument is on channel 1, pipe its direct out line to the auxiliary tape deck's input, and pipe the aux tape deck's output to channel 2, and set channel 2's fader at whichever point produces the desired result. This works best if the auxiliary tape deck is running at 7.5 or 15 ips.
And, sometimes, I'll use both channels on the auxiliary tape deck, so that each note I hit on my guitar (for example) sounds like three.