You can connect to the TimeFactor with a stereo input, but still have it perceived as monophonic content. It just depends how the signal is panned coming out of your synth. Since you are wanting to hook up both mono synths to the TimeFactor, the only way to achieve this would be to submix the two synths using a mixer, and send the stereo output of that mixer to the TimeFactor. You have to make sure that if you're hooking up a mono synth to your Scarlett, that the Scarlett is sending a mono mix out of a stereo output connected to the TimeFactor, not individual Outputs for each synth. Does that make sense?

I just tried it and seems like I always get stereo when DLY Mix is set to midpoint, but when it's not, it really depends on the delay type. Sometimes I get stereo and sometimes not. I assume a lot of delays simply cannot do stereo when only single delay is used or what is the reason

A single delay will not give you stereo, even though if you pan the delayed signal it gives the impression of a bigger space. Most delay algorithms in TimeFactor pan each delay left and right, but as soon as you turn DLY MIX to either side, the image starts to become center-focused. This happens on purpose, to achieve a balanced image field. Otherwise, you'll only hear delays predominantly on one side, which you may want that. In fact, many of the presets use this technique.

To sum it up, for the widest possible stereo image, keep DLY MIX in the middle, and make sure you're not connecting one mono synth to each input. The algorithms are NOT DUAL PATH.