The most succesfull 2-in-1 separate audio component was probably the dual well cassette deck. The World's First High-Speed Dual recoding cassette deck was the ONKYO TA-W800 from 1981. Let's take a look at some audio components that used a different approach for the 2-in-1 concept.
A unique portable radio-cassette recorder with a "slide-in tape 2"
Music Air by SONY.
How about a combination between a reel to reel and a compact cassette machine. This is the unique BRG M 11 from Hungary (1970).
There was of course a 3-in-1 approach too. Reel to reel, compact cassette and 8 track recorder from AKAI X2000D
But what if you build a Dual Reel to Reel?
This is the KORTING MT 3624 from...1964.
You can make vertical standing one too. Dokorder 8010A
Then there was the Bell RT-367
Before the compact cassette - CD combination unit there was also a compact cassette - microcassette combination - Technics M212
Talking about the CD. What if you make a CD player - turntable combination unit. FISHER DAC-9025
More vintage audio at www.1001hifi.com