Does the Fostex allow multi-tracking? Recording WHILE playing unrelated sounds. Plus audio through so they can hear themselves in the mix. I opted for the Korg MR-1000 which is also nice, but it definitely DOESN'T have multi-tracking abilities. i.e. NOT full duplex. Although I can always track continuously and edit out the takes into a timeline. It's just a lot more work and a lot more error prone. The zooms allow multi-tracking and double as audio interfaces. Which is why they're the more common mention.
You ask a good question. To my knowledge, the Fostex does not offer simultaneous playback while recording on a separate channel. At most, you can monitor the incoming channels.
I recorded mono but the Fostex has two XLR powered inputs so you can record in stereo. The portable
Fostex FR2 LE that I used does not offer "time stamping" which I think is your point. In a digital world (and I was once in an analog world so I understand well your concern), time stamping just doesn't matter that much - particularly in audio. For 5 minute digital audio files, synching is not a problem.
In my case, my singer used an iPod with an mp3 file while I recorded her mono in a completely separate, unconnected file. As an amateur, she put in one earplug and as I told her, tightly, she held the other in her hand. I didn't want any noise leakage. Yes, with the Fostex and a good mic, sound leakage is a problem.
I had offered her inexpensive but good Sennheiser HD201 closed headphones (I did this on the cheap) to hear the mp3 files but she didn't like them. As she said, "I can't sing if I can't hear myself." So, she went with one earbud. Thinking back on this, I would choose headphones with a flip up/turn off side. Sound quality doesn't matter much. The singer only needs cues.
I had no problem matching up later her Fostex wav voice recording and a voice-stripped version of the stereo mp3 file on a Sony Vegas timeline. The two files were not out of synch. If anything, it was my singer who missed her cues some times. I solved this through choosing different takes, or stretching/shrinking. Hence, my remark about Glenn Gould.