Cytronex certainly used to, and I believe still does, use the Tongxin as its motor.
My main experience with front hub motors is not recent but I do still own one Tongxin motor - and used to own two more - and am a big fan of them for most applications, but a tandem is one thing I would not use one for. They are particularly good for providing a little bit of help to a reasonably strong rider - rather than doing most of the work for a weak rider. And they are good for lighter riders but not heavy ones.
The Tongxin motor is a thing of beauty - hardly any bigger than a dynamo, virtually silent and with virtually no resistance when not under power. However it has one weakness which is that its split axle and roller drive do not cope well with heavy weight, and there have been multiple failures recorded, where the motor starts to slip after a while, or the axle fails entirely. Also they are known for not being easily repairable.
For a tandem I'd go for something a bit less gorgeous than the Tongxin / Cytronex, but a bit more robust, such as a Bafang, or even a Woosh kit as above, as I'ver heard good things about them. The only thing which would persuade me to go with Cytronex would be if they were to assure me that they believe it would be fine in a tandem - have you asked them? I would ask that question of anyone you think of buying from.
Otherwise, a front hub motor will be fine for a tandem on anything up to about 10% climbs. Steeper than that and it might become difficult for the pilot to be able to get enough weight forward to maintain traction, unless you are good at standing on those types of climbs (we're not) when you might be good up to 12-14%. The extra weight of the hub motor might help the slow speed steering a little bit (which is a different point but front of mind as it is an issue on the tandem I have just come back from riding!).